The most widely used and influential typologies for transsexualism and gender identity disorder (GID) in adolescents and adults employ either sexual orientation or age of onset of GID-related symptoms as bases for categorization. This review compares these two typological approaches, with the goal of determining which one should be employed for the diagnosis of GID in Adolescents or Adults (or its successor diagnosis) in the forthcoming revision of the Diagnostic and Statistical Manual of Mental Disorders (DSM). Typologies based on sexual orientation and age of onset of GID-related symptoms are roughly comparable in ease and reliability of subtype assignment. Typologies based on sexual orientation, however, employ subtypes that are less ambiguous and better suited to objective confirmation and that offer more concise, comprehensive clinical description. Typologies based on sexual orientation are also superior in their ability to predict treatment-related outcomes and comorbid psychopathology and to facilitate research. Commonly expressed objections to typologies based on sexual orientation are unpersuasive when examined closely. The DSM should continue to employ subtypes based on sexual orientation for the diagnosis of GID in Adolescents or Adults or its successor diagnosis.