A classification of obsessive-compulsive phenomena based on the functional definition of obsessions as anxiety/discomfort producing and compulsions as anxiety/discomfort reducing is proposed. Based on the assumption of a functional equivalence between external and internal obsessive-compulsive behavior, four essential obsessive-compulsive forms are derived: (1) obssesions, (2) obsessions plus successful compulsions (anxiety/discomfort reducing), (3) obsessions plus obsessionalized compulsions (anxiety increasing), and (4) rituals consisting mainly of stereotyped, overt or covert behaviors (autonomous of anxiety/discomfort and/or obsessions). In addition, it is hypothesized that forms 1–4 represent developmental stages in the progress of obsessive-compulsive symptomatology, increasing in the order specified in complexity and severity and having varying prognostic implications. Finally, an attempt is made to suggest stage-specific behavioral treatment strategies.