Primary failure of eruption, or primary retention, is an abnormality of tooth eruption that is characterized by cessation of eruption in the pre-eruptive stage, resulting in a deeply embedded tooth in the alveolar bone. This condition should be differentiated from secondary failure of eruption, which implicates termination of the eruptive process following tooth emergence into the oral cavity, resulting in infraclusion associated with impaired growth of the supporting alveolar bone. Primary failure of eruption of primary molars may have several effects on the developing dentition, which include malformation, impaction, or ectopic eruption of the succeeding premolars. Another possible complication is inversion between the premolar and its embedded predecessor primary molar. Primary retention of the primary second molars may result in ectopic eruption of the permanent first molars. This paper reviews the characteristics, etiology, and sequelae of primary failure of eruption of primary molars and describes a case of an unerupted mandibular right primary second molar in a healthy 5-year-old girl. The child had no previous facial trauma or infection and a noncontributory familial history. Treatment consisted of surgical extraction of the embedded molar followed by placement of an intra-alveolar space maintainer and regular follow-up examinations.