This chapter frames the book. It introduces the reader to the antebellum Negro Seamen Acts, a series of laws enacted by Southern state legislatures that barred the ingress of all free black sailors. The laws were enacted to prevent the sailors' "moral contagion" of liberty and autonomy from infecting local slave populations. The Introduction describes how the laws' enactment occurred in lockstep with the solidification of the "positive good" theory of Southern slavery. It outlines how the enforcement of the Seamen Acts unleashed contentiouis debates over state regulatory authority, federal commercial power, and the rights of American and British citizens. These debates were both legal and diplomatic in nature. The chapter argues that, within these legal and diplomatic debates, free black sailors forged a new and robust conceptualization of national citizenship, one anchored in rights and powerful enough to shield individuals from the regulatory authority of state governments.