This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book captures a broad picture of North Korea's military customers in the sanctions era, focusing particularly on what drives them. In doing so it updates the earlier literature and draws trends between known or suspected purchasing states. It presents some of North Korea's 'resilient' customers: a group that is aware of the existence of sanctions, but defines them because of deep political and military ties to Pyongyang, North Korea's reliability as a supplier, shared international isolation and, to some extent, affordability. The book also presents 'ad hoc' customers — states such as the Republic of the Congo and possibly Tanzania — whose awareness of the sanctions regime is weaker than clients in other categories. It examines the different categories of North Korea's customers.