"Modular Arithmetic" is a comprehensive and accessible exploration of a foundational branch of number theory. This book systematically introduces the fundamental concepts of modular arithmetic, beginning with congruence and residue systems, and progressing to advanced topics like the Chinese Remainder Theorem, Fermat's Little Theorem, and their diverse applications. With a focus on clarity and practical relevance, the book provides numerous examples and real-world applications, making the subject accessible to a wide audience, including undergraduate and graduate students in mathematics and related fields. The inclusion of exercises at the end of each chapter enhances the learning experience, allowing readers to reinforce their understanding and skills. Whether readers are seeking a solid foundation in modular arithmetic or are intrigued by its applications in cryptography, coding theory, and beyond, this book serves as a valuable resource for learners, educators, and researchers alike.