Geology of Kasatochi volcano, Aleutian Islands, Alaska, Professional Report 123, shows the distribution of post-2008 volcanic rocks, other uncolidated deposits, and geologic features throughout Kasatochi Island. Kasatochi is a small, isolated island volcano in the center of the Aleutian Island chain. It consists of a roughly circular cone approximately 3 km in diameter with a lake-filled central crater that is 1.2 km in diameter and extends from the highest point on the island to sea level. The oldest unit recognized is a thick series of mid-Pleistocene glaciovolcanic deposits consisting of autobrecciated lava, lahars, and volumetrically minor lava masses that we believe to have been emplaced underneath a regional ice cap. This unit is unconformably overlain by several massive Holocene lavas, above which lies a thick sequence of latest-Holocene pyroclastic deposits likely deposited during the crater-forming eruption. The 2008 eruption enlarged the preexisting crater and produced pyroclastic density currents, surges, and fall that blanketed the entire island except for the crater wall and steep, seaward-facing cliffs on the flanks. The 2008 deposits initially extended the shoreline seaward by up to 500 m. A multidisciplinary effort to document recovery of the ecosystem was initiated, and this study of the geology of the island was undertaken as part of that effort. The dataset contains geologic, structural, stratigraphic, and geochronologic data organized according to the GeMS and AK GeMS mapping schemas. The complete report, geodatabase, and ESRI fonts and style files are available from the DGGS website: http://doi.org/10.14509/29718.