As is well-known, Legendre was the first to state the Law of Quadratic Reciprocity in the form that we now know it (though an equivalent result had earlier been conjectured by Euler), and he was able to prove it in some but not all cases, with the first complete proof being given by Gauss. In this talk we trace the evolution of Legendre's work on quadratic reciprocity in his four great works on number theory, from 1785, 1797, 1808, and 1830.