The beginnings of this project go back at least a decade. At some point I became unhappy with both the popular and professional assumption that America's suburban population has come from the nation's central cities. Clearly, the geographic spread of many urbanized areas did result from the outward mobility of people-primarily whites-from the inner city to the outer city, to the suburban ring, and sometimes to the metropolitan fringe. The unanswered question was: how many suburbanites did not come from the Center City? As the study progressed, other questions arose and came to predominate. These new topics included 1) the changes in suburban population patterns from the decade of the 1950's to the 1960's, 2) the geographic distribution of various metropolitan growth patterns across the nation, and 3) the possibility of developing an historical theory of metropolitan growth from these data. The investigation focused on twenty-eight major metropolitan areas.' The 1960 and 1970 SMSA Census Tract Reports provide data on where people lived five years earlier-i.e., where people lived in 1955 when questioned in 1960, etc. Four useful kinds of responses to this question were reported: percent of people over five years of age living in the same place as before (the Residentially Stable Population), the percentage who previously lived in the Center City of the same metropolitan area (the Center City Migrants), the percentage who lived five years before in some other suburban or fringe location within the same metropolitan area (the Intra-Suburban Migrants),