In this article, the author reviews the characteristics of developmentally appropriate criteria for the identification of early childhood mental health symptoms and disorders and the key components of a comprehensive, empirically based, psychiatric assessment of young children and their families. In the first section, the author discusses the infant/early childhood mental health field's perspectives on mental health and mental health problems in infants, toddlers, and preschoolers. The author then provides an overview of the objections to diagnosis of psychiatric disorders in young children and different approaches to the definition of early childhood psychopathology, including descriptive, dimensional, and categorical approaches. In the second section, the author describes the six essential components of a comprehensive mental health assessment of young children: (1) multiple sessions (2) multiple informants (3) a multidisciplinary approach (4) a multicultural perspective (5) multiple modes of assessment, and (6) a multiaxial diagnostic formulation and treatment plan. The author ends with a discussion of the challenges of diagnosing and assessing mental health symptoms and disorders in children younger than 2 years.