This chapter discusses the distribution of the literature publications among the nine functional somatic syndromes indicating that only the disabilities claimed by patients with fibromyalgia and chronic fatigue syndrome (CFS) have been the object of sustained scientific attention. In both CFS and fibromyalgia, the common disabling symptoms consist of cognitive deficits and exercise intolerance. The psychiatric evaluation serves the purpose of identifying and treating disabling mental disorders that are allowed to coexist with CFS. Neuropsychological disturbances, particularly poor memory and difficulty with tasks that require sustained attention and concentration, are commonly reported among the disabling symptoms of patients with CFS and fibromyalgia. An important task for the disability evaluator is to determine whether the allegedly disabling chronic pain symptom belongs to a functional somatic syndrome or is the defining complaint of a discrete chronic pain syndrome.