The El Niño variability in the equatorial Tropical Pacific is characterized by sea-surface temperature anomalies and associated changes in the atmospheric circulation. Through an enormous monitoring effort over the last decades, the relevant time scales and spatial patterns are fairly well documented. In the meantime, a hierarchy of models has been developed to understand the physics of this phenomenon and to make predictions of future variability. In this review, the robust and relevant details of the observations, the fluid mechanical "building blocks," the theory of the deterministic part of the variability, and the impact of small-scale ("noise") and remote ("external") processes are evaluated.