Functional somatic illness is a clinical concept used to define medically unexplained somatic symptoms considered to express psychological distress. Functional somatic illness may express underlying psychiatric disorders (e.g. fibromyalgia due to non-fearful panic disorder, irritable bowel syndrome due to bipolar disorder). Sustained physiological activation caused by stressful life events combined with catastrophic thinking may be another cause. Functional somatic illness may also be caused by classic conditioning of physiological responses that may have been triggered by biological or emotional stimuli. Operant conditioning may also be a cause. The therapeutic alliance relies on acceptance of the reality of the subjective complaints, without a priori acceptance of the patient's attribution of the cause of the symptoms. We recommend initial exploration of the patient's own ideas about aetiology, including appropriate medical tests. The physician should then change the agenda to a biopsychosocial perspective and identify current stressors and psychosocial variables that reinforce symptoms. Only a few randomised trials have been performed. They suggest that psychological treatment should be systematic and structured, with a focus on information, alternative ways of perception, and problem solving. Active forms of physiotherapy and psychopharmacological drugs may be of some benefit in selected patients.