Retention and relapse are important concepts in orthodontics. Retention is maintaining the corrected tooth positions after treatment, while relapse is the loss of correction. There are several theories on retention, including proper occlusion, apical base positioning, and mandibular incisor positioning. Retainers help maintain corrections and come in removable and fixed varieties. Factors that can lead to relapse include stretched periodontal ligament fibers, growth changes, inadequate bone adaptation, muscle imbalances, untreated etiologies, and third molar eruption. Maintaining corrections long-term may require permanent retention.