This paper presents a framework for designing and evaluating complex interventions to improve health. Complex interventions are those made up of various interconnecting parts, such as stroke units, hospital at home schemes, group psychotherapies, school-based interventions, and cognitive behavioural therapy for depression. Evaluating these interventions is challenging due to difficulties in defining, developing, documenting, and reproducing them. A phased approach is proposed to help researchers define clearly where they are in the research process. The framework includes several phases: preclinical or theoretical, phase I (defining components), phase II (defining trial and intervention design), phase III (methodological issues for main trial), and phase IV (promoting effective implementation). In phase I, qualitative research can help identify the intervention's components and potential barriers. In phase II, the feasibility and acceptability of the intervention are tested, and the control intervention is defined. In phase III, the main trial is designed, considering issues such as sample size, randomisation, and potential biases. In phase IV, the intervention's implementation is examined, including its uptake and possible adverse effects. The paper emphasizes the importance of using an iterative phased approach that combines qualitative and quantitative methods to improve study design, execution, and the generalisability of results. It also highlights the need for outcomes that are relevant to both patients and the health system, including economic measures. The findings of complex intervention trials are more generalisable if they are performed in the setting where they are most likely to be implemented. The paper concludes that trials of complex interventions are increasingly important due to the drive to provide the most cost-effective health care.This paper presents a framework for designing and evaluating complex interventions to improve health. Complex interventions are those made up of various interconnecting parts, such as stroke units, hospital at home schemes, group psychotherapies, school-based interventions, and cognitive behavioural therapy for depression. Evaluating these interventions is challenging due to difficulties in defining, developing, documenting, and reproducing them. A phased approach is proposed to help researchers define clearly where they are in the research process. The framework includes several phases: preclinical or theoretical, phase I (defining components), phase II (defining trial and intervention design), phase III (methodological issues for main trial), and phase IV (promoting effective implementation). In phase I, qualitative research can help identify the intervention's components and potential barriers. In phase II, the feasibility and acceptability of the intervention are tested, and the control intervention is defined. In phase III, the main trial is designed, considering issues such as sample size, randomisation, and potential biases. In phase IV, the intervention's implementation is examined, including its uptake and possible adverse effects. The paper emphasizes the importance of using an iterative phased approach that combines qualitative and quantitative methods to improve study design, execution, and the generalisability of results. It also highlights the need for outcomes that are relevant to both patients and the health system, including economic measures. The findings of complex intervention trials are more generalisable if they are performed in the setting where they are most likely to be implemented. The paper concludes that trials of complex interventions are increasingly important due to the drive to provide the most cost-effective health care.