Throughout history, human beings have been variously occupied with enquiries into self-knowledge and self-understanding. In Western cultures these themes have persistently raised vital and profound questions not just for individuals but, particularly in the last two to three hundred years, for the sciences and humanities. Psychiatry has been no exception to this. However, although as we shall see in this book, interest in insight in psychiatry has a relatively long past, it is only in the last fifteen or twenty years that psychiatry has become engrossed with the empirical question concerning the presence and nature of insight in patients with mental disorders. It is a question that encompasses many facets. From one perspective, it addresses in a practical way the degree of understanding patients have about their conditions. In turn, this raises important issues relating to clinician-patient communication and carries implications for the management of the individual patient. From another perspective, however, the question of patients' insight reaches to the core of our understanding of mental disorders themselves. It forces us to consider, for example, how mental functions might act and interact in health and illness. Can mental disorders have selective effects on mental function? To what extent can mental dysfunction in one area affect mental function or capacity in another area? The question of insight from yet a different perspective is wider still and focuses enquiry on the nature of self in relation to mental illness.