The projected entitled "The Genesis of Racial Segregation in American Cities" aims to understand the fundamental causes of immigrant and African American segregation in U.S. cities in the late nineteenth and early twentieth centuries. For our analysis we are constructing a spatially explicit dataset spanning ten cities between 1800 and 1930. Our microdata, which consists of 60 million records from a genealogy website, are now fully cleaned, and seven of the ten cities are completely mapped for the entire study period. Our current work investigates the discriminatory nature of early land use regulation and decomposes trends in segregation of various racial and ethnic groups. Next year we plan to assess the mechanisms underlying these trends in segregation as well as analyze the racial equity of the Depression era HOLC (Home Owners' Loan Corporation) program.