Recent advances in observational and theoretical studies of El Niño have shed light on controversies concerning the possible effect of global warming on this phenomenon over the past few decades and in the future. El Niño is now understood to be one phase of a natural mode of oscillation-La Niña is the complementary phase-that results from unstable interactions between the tropical Pacific Ocean and the atmosphere. Random disturbances maintain this neutrally stable mode, whose properties depend on the background (time-averaged) climate state. Apparent changes in the properties of El Niño could reflect the importance of random disturbances, but they could also be a consequence of decadal variations of the background state. The possibility that global warming is affecting those variations cannot be excluded.