Immigration enforcement is carried out by a complex legal and administrative system, operating under frequently changing legislative mandates and policy guidance, with authority and funding spread across several agencies in two executive departments and the courts. The U.S. Department of Homeland Security (DHS) is responsible for conducting immigration enforcement both at the border and in the United States; the U.S. Department of Justice (DOJ) is responsible for conducting immigration removal procedures and criminal trials and for prosecuting people charged with immigration-related crimes. DOJ confronts at least five technical challenges to modeling its resource needs for immigration enforcement that are specific to the immigration enforcement system. Despite the inherent limitations, budgeting for immigration enforcement can be improved by changing the method for budgeting. Budgeting for Immigration Enforcement addresses how to improve budgeting for the federal immigration enforcement system, specifically focusing on the parts of that system that are operated and funded by the U.S. Department of Justice (DOJ). The report recommends that DOJ establish policy-level procedures to plan and coordinate policy planning and implementation to improve performance of the immigration enforcement system. The report also recommends that DOJ and DHS accelerate their design of an integrated capacity to track cases and project immigration enforcement activity. Policy makers and others who are interested in how the nation's immigration enforcement system is organized and operates also will find it useful.