ENSO is capable of affecting normal climatic patterns around the world. El Ni o, and its counterpart, La Ni a, modify the normal precipitation patterns in Mexico. In general, during El Ni o (La Ni a) winters, precipitation increases (decreases) over northwestern Mexico, while it decreases (increases) in the region around the Isthmus of Tehuantepec. A southward shift in the position of the subtropical jet stream increases the number of Northerners over the southern part of the Gulf of Mexico. A summer El Ni o causes a deficit in precipitation. Various mechanisms, associated with El Ni o, result in negative precipitation anomalies over most of Mexico. Enhanced subsidence associated with a southward shift of the Inter Tropical Convergence Zone (ITZC), more intense trade winds, a decreased number of tropical cyclones over the Intra Americas Seas (IAS) and reduced relative humidity, may result in severe droughts. These elements produced major socioeconomic loses during the summer of 1997 that could be directly related to El Ni o. During La Ni a years, climate conditions return to normal or result in enhanced precipitation. Current seasonal prediction schemes appear to be skillful enough to provide useful information in the planning of certain socioeconomic activities.