This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book argues for the recognition of noncitizenship as an analytical category as fundamental to the liberal State as citizenship. It examines noncitizenism and the obligations to which activated noncitizenship gives rise. When noncitizenship is not dormant, but 'activated', the book presents how there arises a specific State obligation not to impair, and sometimes to facilitate, individuals' capabilities. It addresses the liberal political thinking in two ways. In the one hand, there is the work of global justice theorists and cosmopolitans who present an expanded liberal theory that, broadly speaking, uses thinking that is usually done on a State level and applies it on the global level. On the other hand, there is work specifically directed towards migration, either focusing on the rights of migrants, or on who has access to citizenship or other statuses.