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.

On April 23, 2010, Arizona enacted S.B. 1070, which is designed to discourage and deter the entry or presence of aliens who lack lawful status under federal immigration law. This report discusses this piece of legislation and some of the notable preemption issues raised by its provisions. Where relevant, it examines the district court's ruling that the federal government is likely to succeed on the merits of its arguments that certain sections of S.B. 1070 are preempted by federal law. It also discusses other preemption issues potentially raised by S.B. 1070 or similar legislation, including some issues that were not expressly addressed by the district court in its preliminary ruling.