A time of fluidity, a time of consolidation, adolescence is fundamentally a time of becoming. It is a period of life most distinguished by energy, creativity and self-consciousness (in all senses of the term) and accompanied by a renegotiation with the world of adults, both in the home and outside of it. This article examines the problematic relationship between diagnosis and adolescent development, looking especially at eating disorders, substance abuse, and psychiatric illness. It describes what is problematic both for the individuals who are diagnosed and in the use of common diagnostic nomenclature for adolescents, who may best be described in different terms from those of children or adults. It concludes with implications for treatment and a suggestion that new categories for adolescent diagnoses may need to be developed to characterize more adequately adolescent disorders.