This book is a reissue of the second edition which appeared in 1940. It has the distinction of being the first vintage mathematical work published in the NCTM series "Classics in Mathematics Education." The text includes a biography of Pythagoras and an account of historical data pertaining to his proposition. The remainder of the book shows 370 different proofs, whose origins range from 900 B.C. to 1940 A.D. They are grouped into the four categories of possible proofs: Algebraic (109 proofs); Geometric (255); Quaternionic (4); and those based on mass and velocity, Dynamic (2). Also included are five Pythagorean magic squares; the formulas of Pythagoras, Plato, Euclid, Maseres, Dickson, and Martin for producing Pythagorean triples; and a bibliography with 123 entries. (RS)