The question "What is mental illness?" raises many issues in many contexts, personal, social, legal, and scientific. This chapter reviews mental health problems as they appear to the person with the problems, and to family and friends-before the person attends the clinic and is given a diagnosis-a time in which whether there really is a problem, as opposed to life's normal troubles and variations, is undecided, as also the nature of the problem, if such it be, and the related matter what kind of expert advice should be sought. Once at the clinic, a diagnosis may be given-using criteria well-worked-out in the diagnostic manuals. The chapter discusses the conceptualizations of mental disorder in the diagnostic manuals, their rationale, and what can and cannot be reasonably expected of them. There are more position statements than definitions, and while they signal many dilemmas, they do not resolve them. Attempts to do so in the surrounding literature on the concept of mental illness are reviewed in the chapter, with conclusions favoring the features emphasized in the diagnostic manuals: distress and impairment. Finally the chapter considers how far the science may help draw boundaries around mental illness.