Geological evidence (geomorphological, sedimentological, paleontological, and archeological) for the occurrence of El Ni¿o events in Peru during the Quaternary has been reevaluated. New data concerning thermally anomalous molluscan assemblages in Holocene deposits of north central Peru are briefly described, as are new data regarding the distribution of warm- and cold-water mollusks in late Pliocene and early Pleistocene deposits of northwestern Peru. No conclusive evidence of prehistoric ''El Ni¿o'' events was found in the new data or the reviewed literature. Most references to El Ni¿o events are best characterized as descriptions of El Ni¿o-like conditions. These conditions may accurately represent events or only mimic events. Past studies lack evidece of two necessary criteria for proof of an El Ni¿o event: a demonstratably short-lived event, and the advection of warm coastal marine waters into the southeastern Pacific Ocean. The glaciological record of precipitation in the Andes, when combined with historical accounts of El Ni¿o phenomena on the coast, is the sole example of a geological data set that meets both criteria. ¿ American Geophysical Union 1987