This two-volume book is a modern introduction to the theory of numbers, emphasizing its connections with other branches of mathematics. It attempts to provide such an understanding of the nature and extent of mathematics. The book covers standard topics in number theory, but also places the whole subject into its historical context. It contains a treasury of proofs, several of which are gems seldom seen in number theory books. Part A is accessible to a first-year undergraduate and deals with elementary number theory. Part B is more advanced and gives the reader an idea of the scope of mathematics today. The connecting theme is the theory of numbers. By exploring its many connections with other branches a broad picture is obtained. This book will appeal to undergraduate students in mathematics and engineering.