In the ?rst section of this opening chapter we review two different proofs of the PythagoreanTheorem,oneduetoEuclidandtheotheroneduetoaformerpresident oftheUnitedStates,JamesGar?eld.Inthesamesectionwealsoreviewsomehigher dimensional analogues of the Pythagorean Theorem. Later in the chapter we de?ne Pythagorean triples; explain what it means for a Pythagorean triple to be primitive; and clarify the relationship between Pythagorean triples and points with rational coordinates on the unit circle. At the end we list the problems that we will be interested in studying in the book. In the notes at the end of the chapter we talk about Pythagoreans and their, sometimes strange, beliefs. We will also brie?y review the history of Pythagorean triples.