Many scientists either working on the El Niño/Southern Oscillation (ENSO) problem or its many applications have not been trained in both the equatorial ocean and atmospheric dynamics necessary to understand it. This book seeks to overcome this difficulty by providing a step by step introduction to ENSO, helping the upper level graduate student or research scientist to learn quickly the ENSO basics and be up to date with the latest ENSO research. The text assumes that the reader has a knowledge of the equations of fluid mechanics on a rotating earth and emphasizes the observations and simple physical explanations of them. Following a history of ENSO and a discussion of ENSO observations in Chapters 1 and 2, Chapters 3-5 consider relevant equatorial ocean dynamics, Chapters 6 and 9 relevant atmospheric dynamics, and Chapters 7 and 8 the main paradigms for how the Pacific Ocean and atmosphere couple together to produce ENSO. Chapter 8 also discusses the old mystery of why ENSO tends to be locked in phase with the seasonal cycle. Successful dynamical and statistical approaches to ENSO prediction are discussed in Chapters 10 and 11 while Chapter 12 concludes the book with examples of how ENSO influences marine and bird life.