Many climatic extremes around the globe, such as severe droughts and floods, can be attributed to the periodic warming of the equatorial Pacific sea surface, termed the El Niño or Southern Oscillation (ENSO). Advances in our understanding of ENSO, in which Edward S. Sarachik and Mark A. Cane have been key participants, have led to marked improvements in our ability to predict its development months or seasons, allowing adaptation to global impacts. This book introduces basic concepts and builds to more detailed theoretical treatments. Chapters on the structure and dynamics of the tropical ocean and atmosphere place ENSO in a broader observational and theoretical context. Chapters on ENSO prediction, past and future, and impacts, introduce broader implications of the phenomenon. This book provides an introduction to all aspects of this most important mode of global climate variability, for research workers and students of all levels in climate science, oceanography and related fields.