This document is a textbook on number theory, authored by Akshansh Chaudhary, a graduate of BITS Pilani, Dubai Campus. It covers various branches of number theory, including elementary, analytical, geometrical, computational, and algebraic number theory. The text discusses concepts such as Pythagorean triplets, perfect numbers, Euclid's algorithm, Diophantine equations, and the fundamental theorem of arithmetic. It also delves into topics like congruences, primitive roots, and quadratic residues. The document is structured to provide a comprehensive understanding of number theory, suitable for educational purposes.