The project successfully increased the number of individuals enumerated in the 1940 U.S. Census of Population to whom a Census Bureau issued protected identification key (PIK) is attached. The PIK is unique for each individual in the U.S. and makes it possible for a researcher in a Federal Statistical Research Data Center (RDC) to easily link records for the same individual across a variety of sources (e.g. locating the same individual in the 1940 census and the 1973 Current Population Survey). The project was able to attach PIKs to 70% of individuals under age 20 in 1940. The biases in the assignment of PIKs was investigated, and strategies for achieving comparably high PIK rates for the population age 20+ in 1940 were developed. These strategies can be implemented as new sources of data containing exact places and dates of birth (e.g. World War Two draft registration cards; birth, marriage, and death records) for large numbers of U.S. residents who were alive in 1940 are digitized. The ability to attach PIKs to the 1940 census improves our ability to link records for the same individual across time, generating longitudinal data from a series of previously unrelated cross-sectional data collections. This will substantially enhance our understanding of how circumstances encountered at one point in the life cycle (e.g. exposure to waterborne lead in childhood, in utero exposure to influenza) lead to outcomes observed many years later in the life cycle (e.g. IQ, educational attainment, adult income, disability, susceptibility to Alzheimer’s Disease), and to the efficacy of public health interventions (clean water, immunization) the full benefits of which are realized only over entire lifetimes or across generations.