"The task is done; the Maker rests. And lo! The engine turns. A million years shall flow, Ere round its axle shall the wheel run slow And a new cog be needed. " Mad8.ch: The Tragedy of Man J.C.W. Horne's translation In this book I tried to sum up the facts and results I considered most important concerning periodic solutions of ordinary differential equations (ODEs) produced by this century from Henri Poincare up to the youngest mathematician appearing in the list of references. I have included also some results of my own that did not find their way into monographs in the past. I have done research in this direction for more than 25 years and have given graduate courses about some of the topics covered for many years at the Budapest University of Technology and also at the Universidad Central de Venezuela in Caracas. I hope that people interested in differential equations and applications may use this experience. Some may say that periodic solutions of ODEs has been a closed chapter of mathematics for some time.