OBJECTIVES: Clinical pathways can improve care and outcomes for children with respiratory illnesses. The Simultaneously Implementing Pathways for Improving Asthma, Pneumonia, and Bronchiolitis Care for Hospitalized Children (SIP) trial is a multicenter, randomized trial of a high-efficiency pathway intervention in general and community hospitals. Our objective was to describe implementation fidelity, strategy use, time costs, barriers, and facilitators. METHODS: We conducted a mixed-methods study. Hospitals received clinical pathways (intervention) and used 5 implementation strategies: quality improvement (QI) mentor meetings, education, iterative changes, audit and feedback, and clinical decision support via electronic order sets. Data were collected through monthly surveys (11 months) of site leaders and recordings of mentor meetings. Quantitative data were analyzed using descriptive statistics, and qualitative data were analyzed using thematic content analysis. RESULTS: Eighteen site leaders (from 18 hospitals) and 8 QI mentors participated. Monthly survey completion rates were 72% to 100%. Pathway implementation fidelity was high (94%). Implementation strategies with the highest use were QI mentor meetings, iterative changes, and electronic order sets. Audit and feedback had the lowest use, driven by information technology challenges and delays in data collection. Implementation time costs were approximately 14 hours per month, and data collection had the highest time cost. Implementation barriers included time limitations and stakeholder resistance to change. Facilitators included SIP study resources, engagement of multidisciplinary staff, and alignment with institutional goals. CONCLUSIONS: Our multicenter study provides detailed guidance on implementation fidelity, strategy use, time costs, barriers, and facilitators for general and community hospitals implementing high-efficiency pediatric pathway interventions.