Despite widespread concern about homelessness, fundamental questions about the size and characteristics of this hard to study population are unresolved, in large part because it is unclear whether existing data are sufficiently complete and reliable. We examine these questions as well as the coverage of new microdata sources that are designed to be nationally representative and will allow pathbreaking new analyses. We compare three restricted use data sources that have been largely unused to study homelessness to less detailed public data. In doing this triangulation of sources, we examine the completeness and accuracy of available data and improve our understanding of the size of the homeless population and its inclusion in household surveys. Specifically, we compare restricted data from the 2010 Census, American Community Survey (ACS), and Homeless Management Information System (HMIS) to HUD’s public-use point-in- time (PIT) estimates and the Housing Inventory Count (HIC) at the national, city and county, and person level. We explore the extent to which definitions, weighting, frame completeness, and seasonality explain discrepancies between sources. We also link HMIS shelter-use data to the Census to evaluate the usefulness of these microdata to study this population. Our analyses suggest that on a given night there are 500,000-600,000 people experiencing homelessness in the U.S., about one-third of whom are sleeping on the streets and two-thirds in shelters. About 80-95 percent of those in shelters were counted in the Census. Despite employing substantially different methods, the Census, ACS, and PIT arrive at similar estimates after accounting for definitional differences, ambiguity in the classification of certain facilities, and differences arising from the timeframe of Census response. The coverage of these sources is surprisingly good given the difficulties of surveying this population. By establishing the broad coverage and reliability of the new data sources, this paper lays the foundation for pathbreaking future work on the characteristics, income, safety net participation, mortality, migration, geographic distribution, and housing status transitions of the U.S. homeless population.