The DC: 0–3 Assessment Protocol Project, an initiative of ZERO-TO-THREE's diagnostic classification task force, was undertaken in response to the need for an assessment strategy that would support DC: 0–3 diagnostic formulation for both clinicians and researchers. The Project Workgroup developed and pilot tested an assessment protocol that would: (1) define a sufficiently comprehensive information set to direct and organize observations for clinicians and researchers to systematically assign diagnoses and evaluate interrater agreement with respect to DC: 0–3 and DMS-IV diagnoses; (2) be flexible enough for use in different settings; and (3) offer potential benefit to participating children and families. The DC: 0–3 Assessment Protocol employs observational strategies, multiple visits, and a multimethod approach to assessment that emphasizes the importance of obtaining information about how a young child's behavior varies across observational contexts and with different caregivers. Based on results of pilot testing the protocol, Workgroup members agreed on the need for futher clarification of the criteria for the diagnoses on Axis I, for differentiating Axis I disorders that reside in the child from Axis II disorders conceptualized as disrupted or disordered relationships, and on the need for refinement of Axis V. Implications for clinical practice, training, and research are discussed. ©2003 Michigan Association for Infant Mental Health.