A clinically informative behavioral literature on chronic fatigue syndrome (CFS) and fibromyalgia (FM) has emerged over the past decade. The purpose of this article is to (a) define these conditions and their less severe counterparts, i.e., unexplained chronic fatigue (UCF) and chronic widespread pain; (b) briefly review the behavioral theory and intervention literature on CFS and FM; and (c) describe a user-friendly clinical model of assessment and intervention for these illnesses. The assessments described will facilitate understanding of the somewhat unusual and puzzling somatic presentations that characterize these patients. Using an individualized cognitive-behavioral approach the mental health clinician can offer significant help to these often stigmatized and medically underserved patients. © 2010 Wiley Periodicals, Inc. J Clin Psychol 66:1–25, 2010.