This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book aims to develop a novel theoretical account of the nature of suffering, which highlights the role suffering plays in transforming the world. It describes a different account, according to which suffering a mental state – such as pain, grief, or guilt – consists in that state’s disrupting the wider mental life. The book also aims to clarify a number of distinctions and relations between pain and suffering, and clarify the conceptual space in which these things are located. It focuses on the project by proposing and defending novel accounts of key phenomena that are closely related to suffering. The book argues that the pleasantness and unpleasantness of the mental states is ‘reason-responsive’. It examines the tension between the idea that pain is necessary for moral agency and meaning-making, but at the same disrupts both of these.