Christiaan Huygens, FRS (14 April 1629 – 8 July 1695) was a prominent Dutch mathematician, astronomer, physicist, horologist, and writer of early science fiction. His work included early telescopic studies elucidating the nature of the rings of Saturn and the discovery of its moon Titan, the invention of the pendulum clock and other investigations in timekeeping, and studies of both optics and the centrifugal force.

Huygens achieved note for his argument that light consists of waves, now known as the Huygens–Fresnel principle, which became instrumental in the understanding of wave-particle duality. He generally receives credit for his discovery of the centrifugal force, the laws for collision of bodies, for his role in the development of modern calculus and his original observations on sound perception. Huygens is seen as the first theoretical physicist as he was the first to use formulae in physics.

Christiaan Huygens was born in April 1629 at The Hague, the second son of Constantijn Huygens, (1596–1687), a friend of mathematician and philosopher René Descartes, and of Suzanna van Baerle (deceased 1637), whom Constantijn had married on 6 April 1627. Christiaan studied law and mathematics at the University of Leiden and the College of Orange in Breda. After a stint as a diplomat, Huygens turned to science.

The Royal Society elected Huygens a member in 1663. In the year 1666 Huygens moved to Paris where he held a position at the French Academy of Sciences under the patronage of Louis XIV. Using the Paris Observatory (completed in 1672) he made further astronomical observations. In 1684 he published "Astroscopia Compendiaria" which presented his new aerial (tubeless) telescope. Huygens moved back to The Hague in 1681 after suffering serious illness. He attempted to return to France in 1685 but the revocation of the Edict of Nantes precluded this move. Huygens died in The Hague on 8 July 1695, and was buried in the Grote Kerk.

After Blaise Pascal encouraged him to do so, Huygens wrote the first book on probability theory, De ratiociniis in ludo aleae ("On Reasoning in Games of Chance"), which he had published in 1657. Huygens formulated what is now known as the second law of motion of Isaac Newton in a quadratic form. Newton reformulated and generalized that law. In 1659 Huygens derived the now well-known formula for the centrifugal force, exerted by an object describing a circular motion. Furthermore, Huygens concluded that Descartes' laws for the elastic collision of two bodies must be wrong and formulated the correct laws.

Huygens is remembered especially for his wave theory of light, expounded in his Traité de la lumière (Huygens-Fresnel principle). The later theory of light by Isaac Newton in his Opticks proposed a different explanation for reflection, refraction and interference of light assuming the existence of light particles. The interference experiments of Thomas Young vindicated Huygens' wave theory in 1801, as the results could no longer be explained with light particles. Huygens experimented with double refraction (birefringence) in Icelandic crystal (calcite) and explained it with his wave theory and polarised light.

He also worked on the design of accurate clocks, suitable for naval navigation. His invention of the pendulum clock, patented in 1657, was a breakthrough in timekeeping. Huygens was a scholar, scientist, and inventor, not a clockmaker, and is not known to have ever made any clock himself; he contracted the construction of his clock designs to Salomon Coster in The Hague, who actually built the first pendulum clocks. In 1673 he published his mathematical analysis of pendulums, Horologium Oscillatorium sive de motu pendulorum, his greatest work on horology. It had been discovered that pendulums are not isochronous for large swings; that is, their period depends on the width of swing. Huygens analysed this problem by finding the shape of the curve down which a mass will slide under the influence of gravity in the same amount of time, regardless of its starting point; the so-called tautochrone problem. By geometrical methods which were an early use of calculus, he showed that this curve is a cycloid, not the circular arc of a pendulum's bob, so pendulums are not isochronous. He also solved the problem of how to calculate the period of a pendulum made of an arbitrarily-shaped swinging rigid body, discovering the center of oscillation and its reciprocal relationship with the pivot point. In the same work, he analysed the conical pendulum, consisting of a weight on a cord moving in a circle, using the concept of centrifugal force. Huygens was the first to derive the formula for the period of an ideal mathematical pendulum. Huygens also observed that two pendulums mounted next to each other on the same support will become synchronized, swinging in opposite directions, which he referred to as "odd sympathy". This was the first observation of what is now called coupled oscillations.

Huygens also developed a balance spring clock more or less contemporaneously with, though separately from, Robert Hooke, and controversy over whose invention was the earlier persisted for centuries. In February 2006, a long-lost copy of Hooke's handwritten notes from several decades' Royal Society meetings was discovered in a cupboard in Hampshire, and the balance-spring controversy appears by evidence contained in those notes to be settled in favor of Hooke's claim.

In 1673, Huygens carried out experiments with internal combustion. Although he designed a basic form of internal combustion engine, fueled by gunpowder, he never successfully built one.

In 1675, Christiaan Huygens patented a pocket watch. He also invented numerous other devices, including a 31 tone to the octave keyboard instrument which made use of his discovery of 31 equal temperament.

In 1655, Huygens proposed that Saturn was surrounded by a solid ring, "a thin, flat ring, nowhere touching, and inclined to the ecliptic." Using a 50 power refracting telescope that he designed himself, Huygens also discovered the first of Saturn's moons, Titan. In the same year he observed and sketched the Orion Nebula. His drawing, the first such known of the Orion nebula, was published in Systema Saturnium in 1659. Using his modern telescope he succeeded in subdividing the nebula into different stars. (The brighter interior of the Orion Nebula bears the name of the Huygens Region in his honour.) He also discovered several interstellar nebulae and some double stars. On May 3, 1661, he observed planet Mercury transit over the Sun, using the telescope of telescope maker Richard Reeves in London together with astronomer Thomas Streete and Richard Reeves.

Christiaan Huygens believed in existence of extraterrestrial life. Prior to his death in 1695, he completed a book entitled Cosmotheoros in which he discussed his notions on extraterrestrial life. Huygens was of the opinion that life on other planets is pretty much similar to that on Earth. He thought that availability of water in liquid form was essential for existence of life and therefore the properties of water should vary from planet to planet, since the kind of water that is found on Earth would instantly freeze on Jupiter and vaporize on Venus. He even reported observing dark and bright spots on the surface of planet Mars and Jupiter. This he explained could only be justified by existence of water and ice on those planets.