This article reviews the definition and practice of early childhood mental health consultation (ECMHC) and its evolution from the monograph Early Childhood Mental Health Consultation (Cohen & Kaufman, 2000) to the present. Key efforts to refine the definition and operationalize ECMHC include an in-depth study of six ECMHC programs, a defined model for effective ECMHC (including a list of consultant activities), and articulation of a set of practice-based principles. This work by Georgetown University Center for Child and Human Development and the contributions of others featured in this issue of Zero to Three continue to move the field forward in establishing the evidence base for ECMHC and the assessment of ECMHC fidelity. (author abstract)