Osteoarthritis at the base of the thumb is a common and extremely disabling condition that severely compromises function of the entire hand. Successful treatment is based on an understanding of the specific anatomy and the unique functional attributes of the human hand and thumb. Preservation of the web space is a priority in nonsurgical care and splinting as well as a principal goal of surgical reconstruction. Exercise regimens are designed to emphasize thenar strengthening to encourage preservation of the web space. Activity analysis and modification are focused on joint protection and the avoidance of positions that will accentuate the pathologic condition of trapeziometacarpal subluxation accompanying the retropulsion of the thumb that occurs with contracture of the web space. Surgical treatment is directed toward restoring the thumb-index web space and stabilizing the newly fabricated basal joint by reconstructing the beak ligament and providing a suitable interposition material. After-care likewise emphasizes restoration of the thumb web space, joint mobilization, and strengthening of the supporting thenar musculature. A well-integrated surgical and therapy team will produce uniformly good functional results in the treatment of this disabling condition at the base of the thumb that differentiates us from our simian ancestors.