The Medical Research Council (MRC) has developed new guidance for process evaluation of complex interventions, which are interventions that involve multiple interacting components and are often implemented across different organisational levels. Process evaluation is essential for understanding how these interventions are implemented, how they work, and how they can be adapted to different contexts. The MRC guidance provides a framework for conducting and reporting process evaluations, which were previously not addressed in earlier MRC guidance. The guidance was developed through a collaborative process involving researchers with expertise in evaluating complex interventions. It draws on literature reviews, case studies, and discussions at conferences and seminars. The guidance is relevant not only to public health interventions but also to complex interventions in other domains such as health services and education. It includes a review of influential theories and frameworks, practical recommendations, and six detailed case studies. The MRC process evaluation framework is based on three themes: implementation, mechanisms, and context. Implementation refers to what is implemented and how, including fidelity and dose. Mechanisms refer to how the intervention produces change, including hypothesised causal pathways. Context refers to how external factors affect implementation and outcomes. The guidance emphasizes the importance of a systematic approach to designing and conducting process evaluations, drawing on clear descriptions of intervention theory and identifying key process questions. It also highlights the need for transparency in reporting relationships with stakeholders and for maintaining independence in evaluation. The guidance includes recommendations for planning, designing, conducting, and reporting process evaluations. It emphasizes the importance of involving stakeholders in the evaluation process, ensuring sufficient expertise and resources, and maintaining effective communication within evaluation teams. It also highlights the importance of using a combination of quantitative and qualitative methods to collect and analyze data. The guidance also provides examples of how process evaluations can be conducted, including the process evaluation of the Welsh national exercise referral scheme. It emphasizes the importance of integrating process and outcome data to understand how interventions work and how they can be adapted to different contexts. The guidance also highlights the importance of reporting findings in a way that is clear and useful for policymakers and practitioners.The Medical Research Council (MRC) has developed new guidance for process evaluation of complex interventions, which are interventions that involve multiple interacting components and are often implemented across different organisational levels. Process evaluation is essential for understanding how these interventions are implemented, how they work, and how they can be adapted to different contexts. The MRC guidance provides a framework for conducting and reporting process evaluations, which were previously not addressed in earlier MRC guidance. The guidance was developed through a collaborative process involving researchers with expertise in evaluating complex interventions. It draws on literature reviews, case studies, and discussions at conferences and seminars. The guidance is relevant not only to public health interventions but also to complex interventions in other domains such as health services and education. It includes a review of influential theories and frameworks, practical recommendations, and six detailed case studies. The MRC process evaluation framework is based on three themes: implementation, mechanisms, and context. Implementation refers to what is implemented and how, including fidelity and dose. Mechanisms refer to how the intervention produces change, including hypothesised causal pathways. Context refers to how external factors affect implementation and outcomes. The guidance emphasizes the importance of a systematic approach to designing and conducting process evaluations, drawing on clear descriptions of intervention theory and identifying key process questions. It also highlights the need for transparency in reporting relationships with stakeholders and for maintaining independence in evaluation. The guidance includes recommendations for planning, designing, conducting, and reporting process evaluations. It emphasizes the importance of involving stakeholders in the evaluation process, ensuring sufficient expertise and resources, and maintaining effective communication within evaluation teams. It also highlights the importance of using a combination of quantitative and qualitative methods to collect and analyze data. The guidance also provides examples of how process evaluations can be conducted, including the process evaluation of the Welsh national exercise referral scheme. It emphasizes the importance of integrating process and outcome data to understand how interventions work and how they can be adapted to different contexts. The guidance also highlights the importance of reporting findings in a way that is clear and useful for policymakers and practitioners.