Immigration policy in the United States reflects multiple goals. First, it serves to reunite families by admitting immigrants who already have family members living in the United States. Second, it seeks to admit workers with specific skills and to fill positions in occupations deemed to be experiencing labor shortages. Third, it attempts to provide a refuge for people who face the risk of political, racial, or religious persecution in their country of origin. Finally, it seeks to ensure diversity by providing admission to people from countries with historically low rates of immigration to the United States. Several categories of permanent and temporary admission have been established to implement those wide-ranging goals. This Congressional Budget Office paper describes who is eligible for the various categories of legal admission and provides the most recent data available about the number of people admitted under each category. The paper also discusses procedures currently used to enforce immigration laws and provides estimates of the number of people who are in the United States illegally.