As furstenberg et al. (1979: 497) suggested over a decade ago, “The enormous promise of quantitative history can only be fulfilled when we are intimately acquainted with the limitations of our sources of information.” This collection of articles follows that suggestion by assessing the limitations of perhaps the most important source of information for social science researchers: the U.S. census. Certainly the degree of underenumeration may seriously affect research in many disciplines. For example, our estimates of migration rates might be inflated. In the process of nominal record linkage, researchers typically assume that those who are not listed in the linked census document out-migrated, when in reality many may simply have been missed by the census taker. As a result, past populations (especially in cities) may appear to have been much more mobile than they actually were. Census underenumeration might also exaggerate voter turnout. If the census in a particular community listed 1,215 eligible voters and election returns showed that 1,150 voted in an election, the voter turnout would be estimated at 95%. However, if just 10% of the eligible voters were not counted by the census taker, the estimated level of voter participation would decline to 85%. Finally, our understanding of the rural economy in the past might be affected by underenumeration. Agricultural production might be overstated if the more successful landowning farmers were more often enumerated and less productive farm tenants were missed. In short, the accuracy of the U.S. census should be a central concern to social science historians.