This book is written for trainers and participants of mathematical contests of all kinds up to the highest level of international competitions, including the IMO and the Putnam Competition. It is also written for high school teachers who are conducting a mathematics club and are looking for ideas and problems for their club; for teachers who want to pose "the problem of the week", "the problem of the month", "the research problem of the year"; and for regular high school teachers who are just looking for ideas to enrich their teaching by some interesting nonroutine problems.