The St. Louis Area Earthquake Hazards Mapping Project is a major urban hazard mapping effort supported in part by the U.S. Geological Survey's (USGS) Earthquake Hazards Program, External Research Support. The goal of the project is to provide state-of-the-art urban seismic hazard maps for the greater St. Louis area in Missouri and Illinois that can be used in land-use planning, public policy making, and private sector decision making. Urban seismic hazard maps that include the effects of local geology have been completed for three initial quadrangles and maps for additional quadrangles are being prepared. Liquefaction potential maps for 12 quadrangles in the Mississippi and Missouri River flood plains have also been completed. Surface mapping and subsurface geological, geophysical, and geotechnical information form a three-dimensional geologic database. Reference profiles were generated from shear-wave velocity (Vs) measurements for the uplands (loess/till) and lowlands (alluvial) portions of the study area. Site amplification ranges (distributions) are then generated by the randomization of the Vs profile, dynamic properties, and appropriate input ground motions and these were used to generate probabilistic and scenario ground motion hazard maps. For PGA and 0.2 s Sa, the resulting urban hazard maps show increased ground motion hazard in the uplands, which are thinly covered by loess. The 30-50 m thick alluvium lowlands show similar ground motion relative to the 2008 USGS national seismic hazard maps. For 1.0 s Sa, the urban seismic hazard maps show the reverse - greater amplification on lowlands soil than upland soils. Holocene alluvial units in river valleys and flood plains are the most susceptible to liquefaction. Late Pleistocene glacio-fluvial outwash has a moderate-to-low susceptibility and upland loess deposits have a very low susceptibility. Because many transportation routes, power, petrochemical and gas transmission lines, population centers, and levee structures exist on the highly susceptible Holocene alluvium, parts of the greater St. Louis area are at significant potential risk from seismically induced liquefaction and related ground deformation. Additionally, where infrastructure transitions from lowlands to uplands are areas of significant potential risk due to rapid spatial changes in the level of ground shaking.