This book is part of the Mathematical Olympiad Series which discusses several aspects related to maths contests, such as algebra, number theory, combinatorics, graph theory and geometry. This book will, in an interesting problem-solving way, explain what probability theory is: its concepts, methods and meanings; particularly, two important concepts -- probability and mathematical expectation (briefly expectation) -- are emphasized. It consists of 65 problems, appended by 107 exercises and their answers.