The aims of this thesis were to investigate the prevalence, familial tendency and aetiological factors of ectopic eruption of the maxillary first permanent molar, to analyse associations between ectopic eruption of the maxillary first permanent molar, to analyse associations between ectopic eruption of the maxillary first permanent molar and other tooth and developmental disturbances and cleft lip and/or palate defects, to make a longitudinal evaluation of the effects of orthodontic treatment in irreversible cases and of the prognosis for the atypically resorbed second primary molars in cases of reversible ectopic eruption and to carry out histological studies of second primary molars with extensive resorptions. The thesis is based on eight investigations. The subjects for the prevalence study consisted of 2,903 children. The main subjects in the clinical studies were the children with ectopic eruption of the maxillary first permanent molar from those children. The methods used were radiographic assessments, biometrics, orthodontic treatment, histology, different statistical methods and clinical assessments. The prevalence of ectopic eruption of the maxillary first permanent molar was found to be 4.3 per cent for the population and 21.8 per cent for cleft children. In sibs to children with ectopic eruption the prevalence was 19.8 per cent, indicating a genetic background. The main aetiological factors were the greater mesial angulation and the greater width of the ectopically erupted first permanent molar. In cases of reversible ectopic eruption the atypically resorbed second primary molars remained to the normal exfoliation time in 90 teeth out of 92. Treatment with cervical headgear in 46 children for an average of 9 months resulted in uprighting of the upper first permanent molars to good occlusion and in about 70 per cent of the children sufficient space for the second premolars. In the long-term follow-up, 10 years after treatment, two forms of discriminant analysis were used to evaluate the long-term treatment effects. The analyses showed a uniformity of the cases, indicating that the effects of the eruption disturbance had been corrected and all negative side effects of the treatment were eliminated.