Developmental disorders have traditionally included primary problems in the individual's development of cognitive, communication, motor, or social skills, with onset of such problems occurring prior to the age of 18 years. In practice, the term "developmentally disabled" may refer to individuals with mental handicaps or pervasive developmental disorders (i.e. autism or Asperger's disorder), as well as those exhibiting developmental problems secondary to communication disorders, physical disabilities, or medical conditions.

As with any individual, a comprehensive assessment is needed to determine overall levels of functioning as well as personal strengths and weaknesses, which is important in planning and implementing programs. Evaluation should therefore include an assessment of cognitive development, speech and language skills, motor skills, adaptive or daily living skills, and social-emotional concerns. Although assessment at an early age is often considered less predictive of long-term intellectual potential, it is preferable to have "at risk" children assessed as young as possible in order to identify those children in need of early intervention programs.

A psychologist can provide diagnostic assessment, consultation, program planning, advocacy, and counselling for the developmentally delayed individual and their family. Assessment is essential, not just at the time of diagnosis, but also to monitor an individual's progress, re-assess needs, and document personal milestones. Consultation is key, as is advocacy.

Counselling is additionally helpful to both the developmentally disabled individual and their family. There is a growing awareness that children and adults with developmental disabilities are susceptible to the same social-emotional problems, and need the same social supports, as individuals in the larger population. Particularly of interest to counsellors are those resources, both personal and external, which contribute to the adjustment of those families who cope effectively and report personal satisfaction. By working with those who are experiencing distress (i.e. marital problems, concerns regarding the impact on siblings) and assisting families in accessing or developing these resources, assistance can be more comprehensive, practical, and successful.