We review recent research on the presentation, nosology and epidemiology of behavioral and emotional psychiatric disorders in preschool children (children ages 2 through 5 years old), focusing on the five most common groups of childhood psychiatric disorders: attention deficit hyperactivity disorders, oppositional defiant and conduct disorders, anxiety disorders, and depressive disorders. We review the various approaches to classifying behavioral and emotional dysregulation in preschoolers and determining the boundaries between normative variation and clinically significant presentations. While highlighting the limitations of the current DSM-IV diagnostic criteria for identifying preschool psychopathology and reviewing alternative diagnostic approaches, we also present evidence supporting the reliability and validity of developmentally appropriate criteria for diagnosing psychiatric disorders in children as young as two years old. Despite the relative lack of research on preschool psychopathology compared with studies of the epidemiology of psychiatric disorders in older children, the current evidence now shows quite convincingly that the rates of the common child psychiatric disorders and the patterns of comorbidity among them in preschoolers are similar to those seen in later childhood. We review the implications of these conclusions for research on the etiology, nosology, and development of early onset of psychiatric disorders, and for targeted treatment, early intervention and prevention with young children.