Retention is a critical phase in orthodontic treatment, essential for maintaining the treatment stability of the results achieved by fixed orthodontic treatment. Massive efforts have been carried out to find the best strategies to retain the teeth in their new position after orthodontic treatment and to avoid relapse. Different retention treatments have also been analyzed with respect to tooth stability and periodontal health. The retention phase may last a year or longer, making the use of retainers particularly important during the first year after the orthodontic treatment. Retainers can be fixed or removable. Each kind has advantages and disadvantages. Fixed retainers are effective and do not need the patient’s cooperation, but they have been criticized for their potential to compromise the periodontal status, due to the accumulation of plaque and calculus on the retainer. Thus, removable retainers have become more common for a number of reasons, including lower costs, better esthetics, and less time consumed in the laboratory, and they can be removed from the mouth and cleaned by the patient. Current thinking is to hold the corrections achieved with a retainer because the exact cause of relapse is not known. The etiology of post-treatment relapse is multifactorial and subject to individual variations. As a result, it is impossible to guarantee absolute post-treatment stability.