This book offers a friendly introduction to number theory from a computational perspective. It starts with the basics, such as divisibility, prime numbers and congruences, and then gradually progresses to more advanced topics like continued fractions, Diophantine equations and cryptography. Abstract ideas are presented in a way that makes them concrete and approachable, with examples that can be explored computationally. Each chapter is thoughtfully structured to build on the previous ones, and the book is organized into seven key parts, each accompanied by exercises (with solutions) that encourage independent problem solving and verification of results. Readers can download the free interactive ebook version to engage directly with live Wolfram Language code. This book is ideal for students, self-learners and anyone curious about the underlying arithmetic of modern mathematics.