This report discusses the authority of state and local law enforcement to assist in the enforcement of federal immigration law through the investigation and arrest of persons believed to have violated such laws. It describes current provisions in federal law that permit state and local police to enforce immigration law directly; analyzes major cases concerning the ability of states and localities to assist in immigration enforcement, including the Supreme Court's ruling in Arizona v. United States; and briefly examines opinions on the issue by the Office of Legal Counsel (OLC) within the Department of Justice. This report does not discuss legal issues raised by state and local measures intended to supplement federal immigration laws through the imposition of additional criminal or civil penalties.