The second edition of the Oxford Handbook of Infant, Toddler, and Preschool Mental Health Assessment offers historical understandings and highlights recent enhancements in the assessment of infant, toddler, and preschool mental health. Authors present empirical findings across a variety of assessment methods and measures developed to capture variability in normative and psychopathological social-emotional development, including assessments of multiple dimensions of temperament, emotion regulation and dysregulation, behavior problems, social-emotional competencies, diagnostic symptoms, and diagnostic conditions. Notable in this edition is the consensus regarding the importance of identifying and assessing the relevant relational contexts in which early development is fostered. Also emphasized is the need for professionals to enhance their cultural awareness and responsiveness. Finally, greater dissemination of evidence-based young child mental health assessment methods is encouraged, which will require training interdisciplinary professionals in the cultural and domain expertise to identify and evaluate children with and at risk for clinically significant psychopathology.