The article discusses the development and evaluation of complex interventions, which are widely used in health services, public health, and social policy areas with significant health implications. The Medical Research Council (MRC) has updated its 2000 framework for evaluating complex interventions, addressing new insights and experiences gained over the past decade. Key issues include the need for early piloting and development, a less linear evaluation process, integration of process and outcome evaluation, and the importance of tailoring interventions to local contexts. The article outlines the revised guidance, emphasizing the importance of understanding the characteristics of complex interventions, their effectiveness in real-world settings, and the methods for developing, evaluating, and implementing them. It also highlights the challenges of feasibility, acceptability, and the need for robust reporting standards. The guidance aims to help researchers, funders, and policymakers make appropriate methodological and practical choices in evaluating complex interventions.The article discusses the development and evaluation of complex interventions, which are widely used in health services, public health, and social policy areas with significant health implications. The Medical Research Council (MRC) has updated its 2000 framework for evaluating complex interventions, addressing new insights and experiences gained over the past decade. Key issues include the need for early piloting and development, a less linear evaluation process, integration of process and outcome evaluation, and the importance of tailoring interventions to local contexts. The article outlines the revised guidance, emphasizing the importance of understanding the characteristics of complex interventions, their effectiveness in real-world settings, and the methods for developing, evaluating, and implementing them. It also highlights the challenges of feasibility, acceptability, and the need for robust reporting standards. The guidance aims to help researchers, funders, and policymakers make appropriate methodological and practical choices in evaluating complex interventions.