Misti volcano in southern Peru has a record of explosive eruptions and a nearby population of over 810,000, making it a hazardous volcano. The city center of Arequipa, Peru's second most populous city, is 15 km from the summit of Misti, and many neighborhoods are closer. As the population increases yearly, the urban boundary continues to move up the south side of the volcano. Many parts of the city are built upon the deposits from Misti's most recent Plinian eruption at ca. 2 ka. The 2 ka Plinian eruption (Volcanic Explosivity Index [VEI] 5) produced a 1.4 km 3 tephra-fall deposit and 0.01 km 3 of pyroclastic-fl ow deposits in ̃2-5 h. Column height varied during the eruption but ascended up to 29 km. Pyroclastic fl ows descended only the south side of the volcano. The tephra fall spread southwest, resulting in ̃20 cm of tephra accumulation in the area now occupied by the city center. The fl owage deposits were previously identifi ed as pyroclastic-fl ow deposits, but new sedimentologic and textural evidence suggests that ̃80% (by volume) of the deposits were emplaced wet and relatively cold. As such, they are lahar deposits. A Neoglacial advance concurrent with the eruption supports evidence for voluminous snow and ice on the edifi ce. Pyroclastic fl ows melted between 0.01 km 3 and 0.04 km 3 of ice and snow on the volcano, triggering lahars that descended the volcano and inundated channels and some interfl uves on the south fl ank. The lahars evolved downstream from proximal debris fl ows to distal hyperconcentrated fl ows, emplacing ̃0.04 km 3 of deposits. Four facies of lahar deposits are present in the channels and another facies occurs on the interfl uves. Such a comprehensive understanding of the 2 ka eruption will help to inform future volcanic hazards assessments. Pyroclastic-fl ow and tephra-fall deposits of the same magnitude could occur again and are useful in hazards assessment. The 2 ka lahars required voluminous water, which is no longer available on the volcano, and, within modern climate conditions, these deposits are not representative of possible future events. Estimations of water available from modern rain and snow suggest that lahars with volumes between 1 × 10 5 m 3 and 3 × 10 6 m 3 are possible. Lahars are more likely if an eruption occurs during a period of high snow accumulation or during subsequent heavy rainfall. Lahars up to 1 × 10 7 m 3 are possible if the Río Chili is dammed during an eruption. Lahar hazard zones generated using these volumes suggest the largest lahars could enter Arequipa. © 2011 The Geological Society of America.