This paper is part of a larger effort to understand why there are differences in the level of occupied and vacant housing units among the 2010 Census, the 2010 American Community Survey (ACS), the Current Population Survey/Housing Vacancy Survey (HVS), and the American Housing Survey (AHS). The specific focus of this paper is to provide a snapshot of research completed to date on factors that might explain differences in the level of vacant and occupied housing units between the 2010 Census and 2010 American Community Survey (ACS). Thus, this paper is not intended to answer all questions or issues concerning these differences. The 2010 ACS 1-year estimate for the gross vacancy rate (GVR) was 13.1 percent compared to 11.4 percent for the 2010 Census. We expect to produce a more comprehensive report on the 2010 Census – ACS differences in 2012 with additional reports to address the differences between the 2010 Census, the HVS and AHS. The goals of these reports are: 1) to understand better why these totals differ and 2) to address particular factors, where possible, that might lead to more consistent results across data collection efforts in the future.