This chapter provides the theoretical underpinnings of adaptation research, adaptation areas and the main analytical framework. Various explanatory attempts exist for assessing adaptation deficits. Reasons why there may be adaptation deficits are considered, including various barriers: structural, social, natural, human and informational. A critique is provided of these explanations, which do not touch upon the root causes and persistence of social vulnerability and incremental adaptation action. As a result, the chapter presents the analytical framework of systemic lock-ins of actors; knowledge, information and expertise; institutions and tools; and (physical) infrastructure all of which contribute to adaptation deficits. The analytical lens of adaptation lock-ins is adjusted to offer a holistic analysis of combined vulnerability and adaptation lock-ins. Lock-ins are then conceptualized from a political ecology perspective, which provides the main theoretical foundation and explanatory angle. The chapter continues by briefly summarizing research on adaptation policymaking and barriers in China and the United States. The chapter synthesizes two key contributions and major gaps this book intends to address in the context of adaptation research. These are: 1) Lock-ins as they relate to political institutions and capacity and 2) the need to explain local adaptation deficits in different political contexts and non-pioneer reasons.