Dental crowding is one of the most common malocclusions. The classification into primary, secondary and tertiary crowding is based on the time of occurrence and etiology. The etiology of tertiary crowding is multifactorial, consisting of several theories. The theory about pressure of third molars opposes the fact that the tertiary crowding occurs in persons with congenitally lack of third molars. The second theory states thats the modern diet leads to the lack of aproximal attrition. The most acceptable theory is about the late mandibular growth in the late adolescent years. The choice of therapy depends on the crowding ratio which is determined by gnatometric analysis. There are dentoalveolar and dento-dental discrepancy. Dentoalveolar discrepancy is estimated by the space analysis, while dento-dental discrepancy is determined by the use of odontomterics, specifically Bolton's analysis. Depending on the results of these analysis, orthodontists decide on the appropriate therapy. The most commonly used are fixed orthodontic appliances with interproximal enamel reduction. At the higher compression ratio, we access to extraction of appropriate teeth. After reaching the ideal occlusion with fixed appliances, it is necessary to continue therapy with a retainer to prevent unwanted relapses.