The article discusses the importance of process evaluation in the design and testing of complex interventions, particularly in public health, health services, and education. The Medical Research Council (MRC) has developed new guidance to provide a framework for conducting and reporting process evaluations. This guidance emphasizes the need for a systematic approach, including clear descriptions of intervention theory and identification of key process questions. The MRC framework focuses on three main themes: implementation, mechanisms, and context. It recommends a feasibility and piloting phase after intervention development, followed by effectiveness evaluation, where the emphasis shifts to assessing fidelity, dose, and context. The article also provides practical recommendations for planning, designing, conducting, and reporting process evaluations, using case studies to illustrate key points. Key considerations include building good relationships with stakeholders, ensuring sufficient resources and expertise, and integrating process and outcome evaluations effectively. The guidance aims to facilitate the planning and conduct of process evaluations, enhancing the understanding of how interventions work in practice and their potential for replication.The article discusses the importance of process evaluation in the design and testing of complex interventions, particularly in public health, health services, and education. The Medical Research Council (MRC) has developed new guidance to provide a framework for conducting and reporting process evaluations. This guidance emphasizes the need for a systematic approach, including clear descriptions of intervention theory and identification of key process questions. The MRC framework focuses on three main themes: implementation, mechanisms, and context. It recommends a feasibility and piloting phase after intervention development, followed by effectiveness evaluation, where the emphasis shifts to assessing fidelity, dose, and context. The article also provides practical recommendations for planning, designing, conducting, and reporting process evaluations, using case studies to illustrate key points. Key considerations include building good relationships with stakeholders, ensuring sufficient resources and expertise, and integrating process and outcome evaluations effectively. The guidance aims to facilitate the planning and conduct of process evaluations, enhancing the understanding of how interventions work in practice and their potential for replication.