Tiberius Claudius Caesar Augustus Germanicus (1 August 10 BC – 13 October AD 54), born Tiberius Claudius Drusus, then Tiberius Claudius Nero Germanicus until his accession, was Roman Emperor from 41 to 54 AD. A member of the Julio-Claudian dynasty, he succeeded his nephew Caligula. The son of Drusus and Antonia Minor, he was born in Lugdunum in Gaul, and was the first emperor to be born outside Italy. He was reportedly afflicted with some type of disability, and his family had virtually excluded him from public office until his consulship with his nephew Caligula in AD 37. Claudius' infirmity may have saved him from the fate of many other Roman nobles during the purges of Tiberius' and Caligula's reigns; potential enemies did not see him as a serious threat to them. His very survival led to his being declared emperor by Praetorian Guard after Caligula's assassination, at which point he was the last adult male of his family. Despite his lack of political experience, Claudius proved an able administrator and a great builder of public works. During his reign, the empire conquered Britain, Thrace, Noricum, Pamphylia, Lycia, and Judea. He took a personal interest in the law, presided at public trials, and issued up to twenty edicts a day. However, he was seen as vulnerable throughout his reign, particularly by the nobility. Claudius was constantly forced to shore up his position; this resulted in the deaths of many senators. Claudius also suffered setbacks in his personal life, one of which may have led to his possible murder. These events damaged his reputation among the ancient writers, though more recent historians have revised this opinion. After his death in 54, his grand-nephew and adopted son Nero succeeded him as emperor.

Claudius was born on 1 August 10 BC, in Lugdunum, Gaul, on the day of the dedication of an altar to Augustus. His parents were Nero Claudius Drusus and Antonia, and he had two older siblings named Germanicus and Livilla. Antonia may have had two other children who died young, as well. His maternal grandparents were Mark Antony and Octavia Minor, Caesar Augustus' sister, and as such he was the great-great grandnephew of Gaius Julius Caesar. His paternal grandparents were Livia, Augustus' third wife, and Tiberius Claudius Nero. During his reign, Claudius revived the rumor that his father Drusus was actually the illegitimate son of Augustus, to give the false appearance that Augustus was Claudius' paternal grandfather.

In 9 BC, Drusus unexpectedly died on campaign in Germania, possibly from illness. Claudius was then left to be raised by his mother, who never remarried. When Claudius' disability became evident, the relationship with his family turned sour. Antonia referred to him as a monster, and used him as a standard for stupidity. She seems to have passed her son off on his grandmother Livia for a number of years. Livia was little kinder, and often sent him short, angry letters of reproof. He was put under the care of a "former mule-driver" to keep him disciplined, under the logic that his condition was due to laziness and a lack of will-power. However, by the time he reached his teenage years his symptoms apparently waned and his family took some notice of his scholarly interests. In AD 7, Livy was hired to tutor him in history, with the assistance of Sulpicius Flavus. He spent a lot of his time with the latter and the philosopher Athenodorus. Augustus, according to a letter, was surprised at the clarity of Claudius' oratory. Expectations about his future began to increase.

Ironically, it was his work as a budding historian that destroyed his early career. According to Vincent Scramuzza and others, Claudius began work on a history of the Civil Wars that was either too truthful or too critical of Octavian. In either case, it was far too early for such an account, and may have only served to remind Augustus that Claudius was Antony's descendant. His mother and grandmother quickly put a stop to it, and this may have proved to them that Claudius was not fit for public office. He could not be trusted to toe the existing party line. When he returned to the narrative later in life, Claudius skipped over the wars of the second triumvirate altogether. But the damage was done, and his family pushed him to the background. When the Arch of Pavia was erected to honor the imperial clan in AD 8, Claudius' name (now Tiberius Claudius Nero Germanicus after his elevation to paterfamilias of Claudii Nerones on the adoption of his brother) was inscribed on the edge — past the deceased princes, Gaius and Lucius, and Germanicus' children. There is some speculation that the inscription was added by Claudius himself decades later, and that he originally did not appear at all.

When Augustus died in AD 14, Claudius — then 23 — appealed to his uncle Tiberius to allow him to begin the cursus honorum. Tiberius, the new emperor, responded by granting Claudius consular ornaments. Claudius requested office once more and was snubbed. Since the new emperor was not any more generous than the old, Claudius gave up hope of public office and retired to a scholarly, private life. Despite the disdain of the imperial family, it seems that from very early on the general public respected Claudius. At Augustus' death, the equites, or knights, chose Claudius to head their delegation. When his house burned down, the Senate demanded it be rebuilt at public expense. They also requested that Claudius be allowed to debate in the senate. Tiberius turned down both motions, but the sentiment remained. During the period immediately after the death of Tiberius' son, Drusus, Claudius was pushed by some quarters as a potential heir. This again suggests the political nature of his exclusion from public life. However, as this was also the period during which the power and terror of the Praetorian Sejanus was at its peak, Claudius chose to downplay this possibility.

After the death of Tiberius the new emperor Caligula (the son of Claudius' brother Germanicus) recognized Claudius to be of some use. He appointed Claudius his co-consul in AD 37 in order to emphasize the memory of Caligula's deceased father Germanicus. Despite this, Caligula relentlessly tormented his uncle: playing practical jokes, charging him enormous sums of money, humiliating him before the Senate, and the like. According to Cassius Dio, as well as a possible surviving portrait, Claudius became very sickly and thin by the end of Caligula's reign, most likely due to stress.

On 24 January, AD 41, Caligula was assassinated by a broad-based conspiracy (including Praetorian commander Cassius Chaerea and several Senators). There is no evidence that Claudius had a direct hand in the assassination, although it has been argued that he knew about the plot — particularly since he left the scene of the crime shortly before his nephew was murdered. However, after the deaths of Caligula's wife and daughter, it became apparent that Cassius intended to go beyond the terms of the conspiracy and wipe out the imperial family. In the chaos following the murder, Claudius witnessed the German guard cut down several uninvolved noblemen, including many of his friends. He fled to the palace to hide. According to tradition, a Praetorian named Gratus found him hiding behind a curtain and suddenly declared him princeps. A section of the guard may have planned in advance to seek out Claudius, perhaps with his approval. They reassured him that they were not one of the battalions looking for revenge. He was spirited away to the Praetorian camp and put under their protection.

The Senate quickly met and began debating a change of government, but this eventually devolved into an argument over which of them would be the new Princeps. When they heard of the Praetorians' claim, they demanded that Claudius be delivered to them for approval, but he refused, sensing the danger that would come with complying. Some historians, particularly Josephus, claim that Claudius was directed in his actions by the Judean King Herod Agrippa. However, an earlier version of events by the same ancient author downplays Agrippa's role — so it is not known how large a hand he had in things. Eventually the Senate was forced to give in and, in return, Claudius pardoned nearly all the assassins.

Claudius took several steps to legitimize his rule against potential usurpers, most of them emphasizing his place within the Julio-Claudian family. He adopted the name "Caesar" as a cognomen — the name still carried great weight with the populace. In order to do so, he dropped the cognomen "Nero" which he had adopted as paterfamilias of the Claudii Nerones when his brother Germanicus was adopted out. While he had never been adopted by Augustus or his successors, he was the grandson of Octavia, and so felt he had the right. He also adopted the name "Augustus" as the two previous emperors had done at their accessions. He kept the honorific "Germanicus" in order to display the connection with his heroic brother. He deified his paternal grandmother Livia in order to highlight her position as wife of the divine Augustus. Claudius frequently used the term "filius Drusi" (son of Drusus) in his titles, in order to remind the people of his legendary father and lay claim to his reputation.

Because he was proclaimed emperor on the initiative of the Praetorian Guard instead of the Senate — the first emperor thus proclaimed — Claudius' repute suffered at the hands of commentators (such as Seneca). Moreover, he was the first Emperor who resorted to bribery as a means to secure army loyalty. Tiberius and Augustus had both left gifts to the army and guard in their wills, and upon Caligula's death the same would have been expected, even if no will existed. Claudius remained grateful to the guard, however, issuing coins with tributes to the praetorians in the early part of his reign.

Under Claudius, the empire underwent its first major expansion since the reign of Augustus. The provinces of Thrace, Noricum, Pamphylia, Lycia, and Judea were annexed under various circumstances during his term. The annexation of Mauretania, begun under Caligula, was completed after the defeat of rebel forces, and the official division of the former client kingdom into two imperial provinces. The most far-reaching conquest was the conquest of Britannia.

In AD 43, Claudius sent Aulus Plautius with four legions to Britain (Britannia) after an appeal from an ousted tribal ally. Britain was an attractive target for Rome because of its material wealth — particularly mines and slaves. It was also a haven for Gallic rebels and the like, and so could not be left alone much longer. Claudius himself travelled to the island after the completion of initial offensives, bringing with him reinforcements and elephants. The latter must have made an impression on the Britons when they were used in the capture of Camulodunum. He left after 16 days, but remained in the provinces for some time. The Senate granted him a triumph for his efforts, as only members of the imperial family were allowed such honours. Claudius later lifted this restriction for some of his conquering generals. He was granted the honorific "Britannicus" but only accepted it on behalf of his son, never using the title himself. When the British general Caractacus was captured in AD 50, Claudius granted him clemency. Caractacus lived out his days on land provided by the Roman state, an unusual end for an enemy commander.

Claudius conducted a census in AD 48 that found 5,984,072 Roman citizens, an increase of around a million since the census conducted at Augustus' death. He had helped increase this number through the foundation of Roman colonies that were granted blanket citizenship. These colonies were often made out of existing communities, especially those with elites who could rally the populace to the Roman cause. Several colonies were placed in new provinces or on the border of the empire in order to secure Roman holdings as quickly as possible.

Claudius personally judged many of the legal cases tried during his reign. Ancient historians have many complaints about this, stating that his judgments were variable and sometimes did not follow the law. He was also easily swayed. Nevertheless, Claudius paid detailed attention to the operation of the judicial system. He extended the summer court session, as well as the winter term, by shortening the traditional breaks. Claudius also made a law requiring plaintiffs to remain in the city while their cases were pending, as defendants had previously been required to do. These measures had the effect of clearing out the docket. The minimum age for jurors was also raised to 25 in order to ensure a more experienced jury pool.

Claudius also settled disputes in the provinces. He freed the island of Rhodes from Roman rule for their good faith and exempted Troy from taxes. Early in his reign, the Greeks and Jews of Alexandria sent him two embassies at once after riots broke out between the two communities. This resulted in the famous "Letter to the Alexandrians", which reaffirmed Jewish rights in the city but also forbade them to move in more families en masse. According to Josephus, he then reaffirmed the rights and freedoms of all the Jews in the empire. An investigator of Claudius' discovered that many old Roman citizens based in the modern city of Trento were not in fact citizens. The emperor issued a declaration that they would be considered to hold citizenship from then on, since to strip them of their status would cause major problems. However, in individual cases, Claudius punished false assumption of citizenship harshly, making it a capital offense. Similarly, any freedmen found to be impersonating equestrians were sold back into slavery.

Numerous edicts were issued throughout Claudius' reign. These were on a number of topics, everything from medical advice to moral judgments. Two famous medical examples are one promoting Yew juice as a cure for snakebite, and another promoting public flatulence for good health. One of the more famous edicts concerned the status of sick slaves. Masters had been abandoning ailing slaves at the temple of Aesculapius to die, and then reclaiming them if they lived. Claudius ruled that slaves who recovered after such treatment would be free. Furthermore, masters who chose to kill slaves rather than take the risk were liable to be charged with murder.

Claudius embarked on many public works throughout his reign, both in the capital and in the provinces. He built two aqueducts, the Aqua Claudia, begun by Caligula, and the Anio Novus. These entered the city in AD 52 and met at the famous Porta Maggiore. He also restored a third, the Aqua Virgo. He paid special attention to transportation. Throughout Italy and the provinces he built roads and canals. Among these was a large canal leading from the Rhine to the sea, as well as a road from Italy to Germany — both begun by his father, Drusus. Closer to Rome, he built a navigable canal on the Tiber, leading to Portus, his new port just north of Ostia. This port was constructed in a semicircle with two moles and a lighthouse at its mouth. The construction also had the effect of reducing flooding in Rome. The port at Ostia was part of Claudius' solution to the constant grain shortages that occurred in winter, after the Roman shipping season. The other part of his solution was to insure the ships of grain merchants who were willing to risk traveling to Egypt in the off-season. He also granted their sailors special privileges, including citizenship and exemption from the Lex Papia-Poppaea, a law that regulated marriage. In addition, he repealed the taxes that Caligula had instituted on food, and further reduced taxes on communities suffering drought or famine. The last part of Claudius' plan was to increase the amount of arable land in Italy. This was to be achieved by draining the Fucine lake, which would have the added benefit of making the nearby river navigable year-round. A tunnel was dug through the lake bed, but the plan was a failure. The tunnel was crooked and not large enough to carry the water, which caused it to back up when opened. The resultant flood washed out a large gladiatorial exhibition held to commemorate the opening, causing Claudius to run for his life along with the other spectators. The draining of the lake was revisited many times in history, including by emperors Trajan and Hadrian, and Holy Roman Emperor Frederick II in the Middle Ages. It was finally achieved by the Prince Torlonia in the 19th century, producing over 160,000 acres (650 km²) of new arable land. He expanded the Claudian tunnel to three times its original size.

Because of the circumstances of his accession, Claudius took great pains to please the Senate. During regular sessions, the emperor sat among the Senate body, speaking in turn. When introducing a law, he sat on a bench between the consuls in his position as Holder of the Power of Tribune (The emperor could not officially serve as a Tribune of the Plebes as he was a Patrician, but it was a power taken by previous rulers). He refused to accept all his predecessors' titles (including Imperator) at the beginning of his reign, preferring to earn them in due course. He allowed the Senate to issue its own bronze coinage for the first time since Augustus. He also put the imperial provinces of Macedonia and Achaea back under Senate control. Claudius set about remodeling the Senate into a more efficient, representative body. He chided the senators about their reluctance to debate bills introduced by himself, as noted in the fragments of a surviving speech:

If you accept these proposals, Conscript Fathers, say so at once and simply, in accordance with your convictions. If you do not accept them, find alternatives, but do so here and now; or if you wish to take time for consideration, take it, provided you do not forget that you must be ready to pronounce your opinion whenever you may be summoned to meet. It ill befits the dignity of the Senate that the consul designate should repeat the phrases of the consuls word for word as his opinion, and that every one else should merely say 'I approve', and that then, after leaving, the assembly should announce 'We debated'.