The assessment and psychosocial treatment of children must consider developmental phase and stage, developmental tasks, the nature of the disorder (does it affect a narrow sector or one developmental line or is there a broad disruption of development?) and biological contributions (including constitution, maturational rate, and factors such as the onset of puberty). Treatments can be distinguished along dimensions, including theoretical emphases, degrees of definition of the treatment, scope, the "patient" (child, family), duration, frequency of contacts, combination with other modalities, etc. Similarly, choice of treatment is based on many factors, including the clinician's expertise, availability of resources, wishes of parents, cost, etc. Future research should be grounded in a theory of child development as well as a theory of therapeutic action. Treatments should be well defined and currently practiced, provided by experts, and arise from realistic situations. The study of therapies bridges basic and applied research and provides data of profound relevance for theories of child development.