Controlling admissions is a core element of state sovereignty, but such control entails the opposing goals of preventing unlawful entries, while facilitating legal flows. These policy goals are inherently in tension, as efforts to identify and interdict illegal entrants inevitably challenges, and may delay, the smooth flow of legitimate travellers. The Supreme Court has long held that Congress has absolute authority to control immigration by establishing rules for the admission, exclusion, and deportation of non-citizens. Thus, while the specific issues in U.S. immigration law have evolved over time, a core policy goal has always been to prevent the entry of aliens who threaten U.S. interests. This book focuses on discussing the inspections conducted at each port of entry and then continues to examine the enforcement of immigration between ports of entry.