There has been increased interest in improving mental health assessment and treatment of young children. However, there are limited data regarding the prevalence of psychiatric disorders among infants and toddlers. The recent development of measures and classification systems that are specific to this age group has enabled researchers to begin to investigate the social, emotional, and behavioral problems of young children. The present study reviews the major research that has been conducted in this area and examines the prevalence of psychiatric diagnoses of infants and toddlers referred to a public community mental health system. The current investigation also compares the frequency of diagnoses of children who have been evaluated based on the Diagnostic and Statistical Manual (4th edition) with another group of children who were diagnosed based on the Diagnostic Classification: 0-3 system. Although there were some differences between the two samples and classification systems, analysis indicated that parent-child relational problems, adjustment difficulties, regulatory problems, and developmental disorders are some of the more common diagnoses prevalent among young children referred for outpatient treatment. As diagnostic measures and classification systems continue to improve, clinicians will have a better understanding of the mental health problems and treatment needs of young children.