OBJECT: Although there are several papers in the literature regarding selection of fusion levels in the adolescent patient, fewer articles pertain to this in the adult patient. The author reviewed his experience and the literature and reports on the choice of fusion levels in the adolescent and adult patient. METHODS: After a review of available data, the author determined that the proximal and distal extent of the fusion should be based on defining curves as either major or minor in the adolescent patient. It is often possible to exclude minor curves from the fusion. Relative Cobb measurement, apical deviation from the plumb line, and apical rotation are the most useful means of distinguishing a major from a minor curve. Otherwise, the proximal and distal extent of a fusion should be performed in such a way that the proximal and distal vertebrae are both neutral and stable (bisected by the center sacral line) postoperatively. Additional segments may need to be included in the adult patient in whom extensive degenerative changes and subluxations are present. The decision of whether to terminate a long fusion at L-5 or the sacrum in an adult degenerative lumbar curve is complex and many factors have to be considered. CONCLUSIONS: Guidelines exist for fusion levels in both adolescent and adult patients. Not all curves require fusion. There are many coronal and sagittal considerations that have to be analyzed when making the final decision.