Designing and evaluating complex interventions to improve healthcare is challenging due to the multifaceted nature of these interventions, which often involve multiple components that act independently and interdependently. The Medical Research Council (MRC) framework was developed to address these challenges by providing a stepwise approach for the development and evaluation of complex interventions. This framework, originally designed for evaluating new drugs, has been influential internationally but requires further development. The authors suggest flexible use of the framework through examples and lessons learned, focusing on preliminary work before a definitive trial. Context is crucial in understanding and evaluating complex interventions. It includes socio-economic factors, health service systems, population characteristics, and how these factors change over time. Understanding context is essential not only for designing interventions but also for assessing their effectiveness in different settings. Researchers must consider the context when designing interventions and when reporting trial results to determine their relevance. Defining and understanding the problem is a key step in developing effective interventions. This involves identifying opportunities for intervention that could lead to meaningful improvements in health or healthcare systems. The process includes mapping out mechanisms and pathways from the intervention to the desired outcomes, adding evidence and data to this map. Conceptualising the problem requires understanding the complexity of health issues, which can vary in levels of complexity. Different health problems may require different approaches, and understanding these differences is essential for effective intervention design. Collecting evidence through various research methods, such as systematic literature reviews, epidemiological research, and expert opinion, is important for quantifying the problem and identifying key modifiable risks. This evidence can inform the development of interventions and their evaluation. Developing an optimal intervention requires a clear understanding of the problem and how the intervention is likely to work. The MRC framework emphasizes the importance of designing, describing, and implementing a well-defined intervention as the most challenging part of evaluating complex interventions. Conceptual approaches, such as conceptual modelling or mapping, can clarify the mechanisms by which an intervention might achieve its aims. These models help in understanding the underlying problem and the intervention's potential effects. The evaluation of complex interventions must consider appropriate outcomes that reflect the intervention's mechanisms. This includes identifying barriers or rate-limiting steps in intervention pathways and optimizing combinations of components in the intervention. The ideal evaluation provides convincing evidence of effectiveness without wasting resources. Key tasks in designing such an evaluation include linking outcomes to the intervention's mechanisms, estimating effect sizes, and considering realistic estimates of recruitment and retention. In conclusion, the design of an intervention depends on understanding the underlying problem and context, the processes involved in optimizing the intervention, and the outcomes appropriate to the intervention mechanism. Defining and understanding the problem, developing and understanding the intervention, and developing and optimizing the evaluation are three substantial tasks that can be conducted simultaneously. The process of development ends with one of three scenarios: the intervention may be unlikely to be cost-effective, it may be sufficiently promising to warrant a definitive evaluation, or it may be implemented if the evidence is strong.Designing and evaluating complex interventions to improve healthcare is challenging due to the multifaceted nature of these interventions, which often involve multiple components that act independently and interdependently. The Medical Research Council (MRC) framework was developed to address these challenges by providing a stepwise approach for the development and evaluation of complex interventions. This framework, originally designed for evaluating new drugs, has been influential internationally but requires further development. The authors suggest flexible use of the framework through examples and lessons learned, focusing on preliminary work before a definitive trial. Context is crucial in understanding and evaluating complex interventions. It includes socio-economic factors, health service systems, population characteristics, and how these factors change over time. Understanding context is essential not only for designing interventions but also for assessing their effectiveness in different settings. Researchers must consider the context when designing interventions and when reporting trial results to determine their relevance. Defining and understanding the problem is a key step in developing effective interventions. This involves identifying opportunities for intervention that could lead to meaningful improvements in health or healthcare systems. The process includes mapping out mechanisms and pathways from the intervention to the desired outcomes, adding evidence and data to this map. Conceptualising the problem requires understanding the complexity of health issues, which can vary in levels of complexity. Different health problems may require different approaches, and understanding these differences is essential for effective intervention design. Collecting evidence through various research methods, such as systematic literature reviews, epidemiological research, and expert opinion, is important for quantifying the problem and identifying key modifiable risks. This evidence can inform the development of interventions and their evaluation. Developing an optimal intervention requires a clear understanding of the problem and how the intervention is likely to work. The MRC framework emphasizes the importance of designing, describing, and implementing a well-defined intervention as the most challenging part of evaluating complex interventions. Conceptual approaches, such as conceptual modelling or mapping, can clarify the mechanisms by which an intervention might achieve its aims. These models help in understanding the underlying problem and the intervention's potential effects. The evaluation of complex interventions must consider appropriate outcomes that reflect the intervention's mechanisms. This includes identifying barriers or rate-limiting steps in intervention pathways and optimizing combinations of components in the intervention. The ideal evaluation provides convincing evidence of effectiveness without wasting resources. Key tasks in designing such an evaluation include linking outcomes to the intervention's mechanisms, estimating effect sizes, and considering realistic estimates of recruitment and retention. In conclusion, the design of an intervention depends on understanding the underlying problem and context, the processes involved in optimizing the intervention, and the outcomes appropriate to the intervention mechanism. Defining and understanding the problem, developing and understanding the intervention, and developing and optimizing the evaluation are three substantial tasks that can be conducted simultaneously. The process of development ends with one of three scenarios: the intervention may be unlikely to be cost-effective, it may be sufficiently promising to warrant a definitive evaluation, or it may be implemented if the evidence is strong.