This book gathers carefully chosen selection tests proposed to IMO (International Mathematical Olympiad) teams across many countries. Offering a blend of original solutions and adaptations by the author, this work is chronologically organized and provides a unique insight into the evolution of this mathematical contest. The proposed problems touch on topics such as the Chinese remainder theorem, Diophantine equations, Fermat's theorem, Euler's theorem, perfect squares, sequences of integers, and Pythagorean triples, to name a few. A meticulously crafted index helps the reader navigate through the topics with ease. This book serves as an invaluable preparation tool for both aspiring students and those passionate about mathematics alike.