A summary of the most important results in the existence and stability of periodic solutions for ordinary differential equations achieved in the twentieth century, along with relevant applications. It differs from standard classical texts on non-linear oscillations in that it also contains linear theory; theorems are proved with mathematical rigor; and, besides the classical applications such as Van der Pol's, Linard's and Duffing's equations, most applications come from biomathematics. For graduate and Ph.D students in mathematics, physics, engineering, and biology, and as a standard reference for use by researchers in the field of dynamical systems and their applications.