The Immigration and Nationality Act (INA) authorizes and in some cases requires the Department of Homeland Security (DHS) to detain non-U.S. nationals (aliens) arrested for immigration violations that render them removable from the United States. An alien may be subject to detention pending an administrative determination as to whether the alien should be removed, and, if subject to a final order of removal, pending efforts to secure the alien's removal from the United States. The immigration detention scheme is multifaceted, with different rules that turn on several factors, such as whether the alien is seeking admission into the United States or has been lawfully admitted into the country; whether the alien has engaged in certain proscribed conduct; and whether the alien has been issued a final order of removal. In many instances DHS maintains discretion to release an alien from custody. But in some instances, such as when an alien has committed specified crimes, the governing statutes have been understood to allow release from detention only in limited circumstances. This book focuses on current topics concerning immigration detention and enforcement.