The spectrum of depression is much wider than that reflected in the current diagnostic nomenclature. A large proportion of subjects with depression both in treatment and in the community fail to meet diagnostic criteria for either major depressive disorder (MDD) or dysthymia. Inclusion of subthreshold categories of depression dramatically improves the coverage of treated depression, particularly in community samples, and better enables the characterization of its longitudinal course. This paper investigates the application of diagnostic criteria for both threshold and subthreshold categories of depression in a prospective longitudinal community study of young adults from Zurich, Switzerland. We present the prevalence and treatment rates of each of the depressive subtypes, the degree of diagnostic overlap and the longitudinal stability of subthreshold and threshold categories of depression. The findings indicate that the prevalence rates of subthreshold categories of depression are quite high in the community, and that a substantial proportion of subthreshold depressives, particularly those with recurrent depression, receive treatment. There is a strong tendency for individuals to meet multiple depressive subtypes over time, with little stability of individual categories among those who continue to manifest depression over a 15-year period. The prospective longitudinal data reveal that major depression is both an antecedent to and sequela of subthreshold categories, providing evidence for the validity of the spectrum concept of depression. However, the need for a threshold for the symptom criteria is suggested by the lack of predictive value of minor depression and depressive symptoms only. These result suggest that both the current symptom threshold for a depressive syndrome and recurrence, but not the minimum duration of depressive episodes, are important components of the classification of depression.