BACKGROUND: Despite the well-documented risks to patient safety associated with transitions from one care setting to another, health care organizations struggle to identify which interventions to implement. Multiple strategies are often needed, and studying the effectiveness of these complex interventions is challenging. OBJECTIVE: The objective of this study was to present lessons learned in implementing and evaluating complex transitional care interventions in routine clinical care. RESEARCH DESIGN: Nine transitional care study teams share important common lessons in designing complex interventions with stakeholder engagement, implementation, and evaluation under pragmatic conditions (ie, using only existing resources), and disseminating findings in outlets that reach policy makers and the people who could ultimately benefit from the research. RESULTS: Lessons learned serve as a guide for future studies in 3 areas: (1) Delineating the function (intended purpose) versus form (prespecified modes of delivery of the intervention); (2) Evaluating both the processes supporting implementation and the impact of adaptations; and (3) Engaging stakeholders in the design and delivery of the intervention and dissemination of study results. CONCLUSION: These lessons can help guide future pragmatic studies of care transitions.