The casebook is a compilation of infant mental health case studies as a companion volume to DC:0–5™: Diagnostic Classification of Mental Health and Developmental Disorders of Infancy and Early Childhood (DC:0–5™; ZERO TO THREE, 2016, 2021). The casebook is designed to strengthen understanding of clinical disorders in infancy and early childhood, and to help clinicians better understand the application of the multiaxial framework and culturally sensitive/relational approach to diagnosis that is fundamental to DC:0–5™. Each infant mental health case study provides case conceptualization, diagnostic summary, discussion, and some questions for the reader to reflect on related to the case and clinical issues raised.