The theory of water waves has been a source of intriguing and oftendifficult mathematical problems for at least 150 years. This text briefly discusses fluid mechanics. It then considers the classical problems in linear and non-linear water-wave theory, as well as more modern aspects -- problems that give rise to soliton-type equations. Lastly it examines the effects of viscosity. All the mathematical developments are presented in a straightforward manner. Exercises, further reading, and historical notes make this an ideal text for graduate students.