This book uniquely discusses an approach to illnesses for which the causes are unknown. It fills the gaps in the literature by relaying research on functional illness, identifying and assessing various options, and indicating some decision-making suggestions that should help clinicians and patients think about therapy. Special considerations for functional illness include “probability” explanations based on the experience of others, serious consideration of the severity of symptoms before treatments are applied, and balancing the risks, costs, and consequences of treatment; including the patient’s thoughts and attitudes is critical for success in treating functional illness. Unique and comprehensive, Functional Illness is an invaluable reference for medical physicians of all specialties who wish to understand better and identify functional illness symptoms.