The term “diagnosis” can refer to the name of a disease that afflicts a person or to the process of determining a diagnosis in the first sense. Complex philosophical, metaphysical, epistemological, normative, and logical issues permeate all aspects of diagnosis, beginning with the question of what is being diagnosed. The nature of disease is philosophically controverted. Both ontological and physiological conceptions of disease continue to influence thinking about disease and hence how to diagnose it. The question of whether the concept of disease is essentially value-laden remains open. Diagnosis presupposes some classification of diseases, known as a nosology, in order to distinguish one disease from another. There are many possible ways to classify diseases, and nosologies can have different goals, for example, providing a basis for rational treatment and prognosis, enabling statistical reporting, fostering research, and for administrative aspects of health care. The major elements of the diagnostic process include the history of the illness, the physical examination, and various kinds of laboratory and clinical testing. Each of these elements requires interpretation and is influenced by philosophical presuppositions. Diagnostic reasoning makes use of probabilistic, causal, and deterministic models. It is fundamentally a process of hypothesis formation about possible diagnoses (differential diagnosis) and the systematic confirmation and ruling out of possibilities until one diagnosis is judged best to explain all the data. Diagnosing disease is important for a broad array of medical and social reasons.