As the infant mental health field has turned its focus to the presentation, course, and treatment of clinically significant mental health disorders, the need for reliable and valid criteria for identifying and assessing mental health symptoms and disorders in early childhood has become urgent. In this article we offer a critical perspective on diagnostic classification of mental health disorders in young children. We place the issue of early childhood diagnosis within the context of classification of psychopathology at other ages and describe, in some detail, diagnostic classifications that have been developed specifically for young children, including the Diagnostic Classification of Mental Health and Developmental Disorders of Infancy and Early Childhood (DC:0-3R; ZERO TO THREE, 2005), a diagnostic classification for mental health symptoms and disorders in infants, toddlers, and preschoolers. We briefly outline the role of diagnostic classification in clinical assessment and treatment planning. Last, we review the limitations of current approaches to the diagnostic classification of mental health disorders in young children.