Euclid was an ancient Greek mathematician active in the early Hellenistic period, around 300 BCE. He worked in Alexandria, Egypt, during the reign of King Ptolemy I, and founded a mathematical school there. Very few reliable details about his personal life, birthplace, or education are known.

He is the author of several major mathematical treatises. His most famous and influential work is the Elements, a systematic compilation of the geometric knowledge of his time. Other significant surviving works include the Data (on geometric problems), the Phaenomena (on spherical astronomy), and the Optics (on perspective). The authorship of some works, like the Catoptrics on mirrors, is sometimes disputed. He also wrote several important works that are now lost, such as the Conics and Porisms.

Euclid's historical importance stems from his synthesis of existing mathematics into a rigorous logical system. The Elements is one of the most successful textbooks ever written, serving as the foundational text for geometry for over two thousand years. Its method of starting from axioms and definitions and proceeding by deductive proof became a model for scientific and philosophical exposition. Translated into Arabic and later Latin, it was central to education from the medieval period through the Renaissance and profoundly influenced later thinkers like Isaac Newton. While much of the content was not original, his achievement was in its masterful organization and proof.