Pediatric settings are increasingly called upon to implement early childhood developmental and behavioral health screening as an early identification and health promotion strategy. Understanding the dynamic barriers and facilitators of implementation at various stages will help implementers plan for and address these factors in support of high quality implementation. Our research supported this goal by analyzing longitudinal, qualitative data. There were 128 semistructured interviews conducted with pediatric clinic and implementation providers across four years. Interviews were transcribed, coded, and synthesized using rigorous qualitative methods. Results were produced using an iterative process to summarize, analyze, and consolidate themes about screening implementation over time. Barriers and facilitators of implementation included characteristics of screening implementation as well as contextual characteristics of the pediatric primary care setting. Some implementation themes were stable over time whereas others demonstrated shifts. Results are discussed in terms of lessons learned for successfully integrating this critical preventive practice within pediatric clinics.