Computer scientists of all disciplines must understand the fundamental concepts, methods, and procedures underlying the development and application of information and communication technologies, and be able to apply them in problem-solving. This book presents the algebraic and number-theoretic foundations for this and applies them to practical problem scenarios, such as modular arithmetic, prime number tests, and encryption. The understanding of terms and their interrelationships is supported, among other things, by learning objectives, integrated exercises with sample solutions, and marginal notes. The book is well-suited for self-study.