Claudius Ptolemaeus worked in Egypt during the second century a.d. References in his own writings indicate that he was active as a scientist from the 120's, and he may have lived well into the second half of the century. He seems to have lived in Alexandria, and probably also at Canopus. His reputation as one of antiquity's finest scientists rests principally on his astronomical writings, and especially on the magnificent treatise whose original Greek title was Mathematike Syntaxis (‘Mathematical Treatise’). This work seems later to have been nicknamed Megiste (‘Greatest’) Syntaxis: a transmutation of the latter, expression through Arabic (perhaps first through Persian) and into mediaeval Latin gave it the name Almagest, by which it is now best known. Hand in hand with astronomy went astrology, a field to which Ptolemy's so-called Tetrabiblos brought an intriguing mixture of rationally grounded conjecture and scientific caution. He was an original and influential geographer. He wrote essays in mechanics and in optics, and a monograph called On the Criterion, which discusses the foundations of scientific knowledge and the methods by which it is to be pursued.

He is also the author of a substantial and systematic treatise on harmonics, certainly written at a time when his main researches in astronomy were already well advanced or complete. Though musicologists acknowledge its merits and originality, and though the influence of some of its ideas on Byzantine, mediaeval and Renaissance theorists is widely recognised, no English translation of the whole work has previously been published, and students of the ancient sciences have seldom given it the attention it deserves.