Information about the unauthorized resident population is needed to develop and evaluate US immigration policy, determine the social and economic effects of unauthorized immigration, and assist public and private service providers in carrying out their missions. Until recently, estimates have been available only for selected data points at the national and sometimes the state level. The Center for Migration Studies (CMS) convened a meeting in September 2013 to assess the need for information about the unauthorized resident population. The meeting included leading academics, researchers, nongovernmental organizations (NGOs) that serve immigrants, and local, state, and federal government representatives. Based on the recommendations from that meeting, CMS initiated a project to derive estimates of the size and characteristics of the unauthorized population at the national, state, and sub-state levels, and to make the information readily available to a wide cross-section of users. A series of statistical procedures were developed to derive estimates based on microdata collected by the US Census Bureau in the 2010 American Community Survey (ACS). The estimates provide detailed demographic information for unauthorized residents in population units as small as 100,000 persons. Overall, the estimates are consistent with the limited information produced by residual estimation techniques. A primary consideration in constructing the estimates was to protect the privacy of ACS respondents.