This article provides guidance for researchers developing complex interventions to improve health and healthcare. It outlines key principles and actions for intervention development, emphasizing the importance of a dynamic, iterative process involving stakeholders, reviewing published research, drawing on existing theories, articulating programme theory, undertaking primary data collection, understanding context, and designing and refining interventions through iterative cycles. The guidance is based on a consensus study involving researchers, stakeholders, and an international expert panel. It highlights the need for flexibility in using the guidance, as some actions may not be relevant to all interventions or contexts. The article also discusses the importance of considering future implementation of the intervention in the real world, ensuring that interventions are not only effective but also feasible, acceptable, and sustainable. Key actions for developers include planning the development process, understanding the problem, identifying resources (time and funding), deciding on an approach to intervention development, involving stakeholders throughout the process, bringing together a team and establishing decision-making processes, reviewing published research evidence, drawing on existing theories, articulating programme theory, undertaking primary data collection, understanding the context, paying attention to future implementation, and designing and refining the intervention. The guidance also emphasizes the importance of publishing the intervention development process to facilitate learning and reduce research waste. It is recommended that developers consider the relevance and importance of each action to their specific intervention and context, both at the start and throughout the development process. The guidance is presented as an accessible communication article, not as a research article, to maximize its use as guidance.This article provides guidance for researchers developing complex interventions to improve health and healthcare. It outlines key principles and actions for intervention development, emphasizing the importance of a dynamic, iterative process involving stakeholders, reviewing published research, drawing on existing theories, articulating programme theory, undertaking primary data collection, understanding context, and designing and refining interventions through iterative cycles. The guidance is based on a consensus study involving researchers, stakeholders, and an international expert panel. It highlights the need for flexibility in using the guidance, as some actions may not be relevant to all interventions or contexts. The article also discusses the importance of considering future implementation of the intervention in the real world, ensuring that interventions are not only effective but also feasible, acceptable, and sustainable. Key actions for developers include planning the development process, understanding the problem, identifying resources (time and funding), deciding on an approach to intervention development, involving stakeholders throughout the process, bringing together a team and establishing decision-making processes, reviewing published research evidence, drawing on existing theories, articulating programme theory, undertaking primary data collection, understanding the context, paying attention to future implementation, and designing and refining the intervention. The guidance also emphasizes the importance of publishing the intervention development process to facilitate learning and reduce research waste. It is recommended that developers consider the relevance and importance of each action to their specific intervention and context, both at the start and throughout the development process. The guidance is presented as an accessible communication article, not as a research article, to maximize its use as guidance.