A methodological survey of statistics on internal migration used in the new enlarged edition ofHistorical Statistics of the United States. No one body of materials gives a complete picture but much can be pieced out from the essentially complementary series that exist: the state-of-birth data of the decennial censuses, 1850–; the special migration reports of the 1940 and 1950 censuses; the annual migration estimates of the Current Population Survey, 1947–; and the Department of Agriculture's annual estimates of migration to and from farms, 1920–. An additional important series can be derived from census age-sex-color-nativity distributions by survival methods. All such estimates are inexact but important migration streams are clearly delineated and the margin of error is probably less than that of most economic series to which they are related.