For more than 35 years, research in the Department of Orthodontics, University of Washington has focused on a growing collection of over 600 sets of patient records to assess stability and failure of orthodontic treatment. All had completed treatment a decade or more prior to the last set of data. Evaluation of treated premolar extraction cases, treated non-extraction cases with generalized spacing, cases treated by arch enlargement strategies, and untreated normal occlusions demonstrate similar physiological changes. 1. Arch length reduces following orthodontic treatment, but also does so in untreated normal occlusions. 2. Arch width measured across the mandibular canine teeth typically reduces post-treatment whether the case was expanded during treatment or not. 3. Mandibular anterior crowding during the post-treatment phase is a continuing phenomenon well into the 20–40 age bracket and likely beyond. 4. Third molar absence or presence, impacted or fully erupted, seems to have little effect on the occurrence or degree of relapse. 5. The degree of post-retention anterior crowding is both unpredictable and variable and no pretreatment variables either from clinical findings, casts, or cephalometric radiographs before or after treatment seem to be useful predictors.