Number Theory has fascinated mathematicians from the most ancient of times. A remarkable feature of number theory is the fact that there is something in it for everyone—from puzzle enthusiasts, problem solvers and amateur mathematicians to professional scientists and technologists. This book offers the reader some articles in number theory that appeared in Resonance over the years 1996–2001. The articles included within form a varied lot, beginning with a puzzle, “find four positive integers such that the sum of any two is a square,” to an expository article on one of the great mathematical achievements of the 20th century—the proof of “Fermat’s Last Theorem”.