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<strong class="journal-contentHeaderColor">Abstract.</strong> Overshooting cloud tops (OT) form in deep convective storms when strong updrafts overshoot the tropopause. An OT is a well-known indicator for convective updrafts and severe weather conditions. Here, we develop an OT detection algorithm using thermal IR channels and apply this algorithm to about 20-year MODIS data from both Terra and Aqua satellites to form an extensive, near global climatology of OT occurrences. The algorithm is based on a logistic model which is trained using A-Train observations. We demonstrate that the overall accuracy of our approach is about 0.9 when the probability of the OT candidates is larger than 0.9. The OT climatology reveals a pattern that follows the climatology of deep convection, as well as shallow convection over the mid-latitude oceans during winter cold air outbreaks. OTs appear most frequently over the Intertropical Convergence Zone (ITCZ), central and southeast North America, tropical and subtropical South America, southeast and south Asia, tropical and subtropical Africa, and northern middle-high latitudes. OT spatial distributions show strong seasonal and diurnal variabilities. Seasonal OT variations shift with large-scale climate systems such as the ITCZ and local monsoonal systems, including the South Asian Monsoon, North American Monsoon and West African Monsoon. OT diurnal variations agree with the known diurnal cycle of convection: Maximum OT occurrences are in the afternoon over most land area and around midnight over ocean; and the OT diurnal cycle is stronger and more varied over land than over ocean. OTs over land are usually colder than over ocean except around 10:30 am. The top 10 coldest OTs from both Terra and Aqua mostly occur over land and at night. This study provides OT climatology for the first time derived from two-decade MODIS data that represents the longest and stable satellite records.