This chapter come to grips with diseases, injuries, and impairments as well as with mental disorders. An attempt is made to define those conditions, how they are being construed and some of the features, which constitute them, though most of them are provisional conventions. Diseases are clusters of signs and symptoms. An important distinction is being made between mere manifestational or purely observational disorders, and single-criterion, usually causal disorders, i.e. disorders defined by their causes. Mental disorders differ from so-called physical diseases in the sense that their limits are broader than those of the affected body. Considering a given disease each patient shares characteristics with many but not all the others. The categories of diseases are neither mutually exclusive, not jointly exhaustive and there are neither necessary, nor sufficient conditions for belonging to those classifications. What’s more, what distinguishes physical from psychological medicine is not some ontological difference between body and mind, but that we grasp mental disorders in terms of reasons rather than causes, through a dialogue with the patient. The term disease intends to refer to a provisional, conventional, useful, and discrete pattern of organic or mental, statistically recurring pathologic processes or states, undergone by an individual, acknowledged by medical science, and which has variable degrees of severity, namely intrinsic suffering, incapacity and/or mortality. The need for medical intervention is constitutive of the concept of disease. Diseases have known or unknown causes, and a natural history, and they are intended to be mutually exclusive. Diseases embody a need for prevention, remedy, or appropriate care. Diseases are neither natural kinds, nor social constructs, inasmuch as they are provisional, shifting, and conventional medical constructs. Ill persons are grouped into overlapping categories, which provide a whole spectrum coming in all grades of severity and not a dichotomy, and that are believed to have utility in the management of their illness and in the circumstances that led to it.