Classification and definition of criteria for eating disorders (ED) have been modified in the last decades due to changing theories about these disorders. The academic approach has been transformed from time to time by results from studies with increasing number of subjects and updated psychological tools. Investigation of EDs means a great challenge for researchers since they constitute a heterogeneous group and exhibit a dynamically variable manifestation and chronological progress. The author provides an overview on the different theories and related most significant results. Despite the intensively developing research in this field, further investigations are needed for the exact determination of subclusters of EDs and to develop successful treatment protocols.