El Nino refers to the exceptionally warm sea temperatures in the tropical Pacific, but it is linked to major changes in the atmosphere through the phenomenon known as the Southern Oscillation (SO), so that the whole phenomenon is called El Nino–Southern Oscillation (ENSO) by scientists. The phenomenon arises because of the coupled atmosphere–ocean interactions whereby the winds create changes in sea temperatures that in turn determine where the main tropical convection and storm activity occur, and thus determine the winds. This article outlines the current understanding of ENSO and the physical connections between the tropical Pacific and the rest of the world, using the 1997–98 El Nino event as a particular illustration. The mechanisms are described along with the interannual variations in climate over time and the associated El Nino events. The impacts are described especially in terms of the associated floods and droughts around the world. An update is included to encompass the super El Nino event of 2015–16.