Knowledge of the structure of the number system and fundamental principles of number theory is essential foundational knowledge in mathematics. This book covers a wide range of topics, from calculations with natural numbers to properties of divisibility and congruence considerations, leading to number-theoretic functions and applications such as cryptography and number encoding. Emphasis is placed on a clear and comprehensive presentation of the material. Proof ideas that underpin rigorously conducted proofs and the connection of specialized knowledge with school-related references are highlighted as distinctive features. The presentation is supplemented by numerous exercises, complete with hints and full solutions.