This article provides guidance for researchers on developing complex interventions to improve health and healthcare. The authors, based on a consensus study informed by reviews and qualitative interviews, present key principles and actions for intervention development. These include viewing intervention development as a dynamic, iterative process involving stakeholders, reviewing published research evidence, drawing on existing theories, articulating program theory, conducting primary data collection, understanding context, and designing and refining the intervention using iterative cycles of development with stakeholder input. The guidance emphasizes the importance of considering each action's relevance to a specific intervention in a specific context, both at the start and throughout the development process. The article also highlights the need for flexibility in applying the guidance, as not all aspects may be relevant to every intervention or context.This article provides guidance for researchers on developing complex interventions to improve health and healthcare. The authors, based on a consensus study informed by reviews and qualitative interviews, present key principles and actions for intervention development. These include viewing intervention development as a dynamic, iterative process involving stakeholders, reviewing published research evidence, drawing on existing theories, articulating program theory, conducting primary data collection, understanding context, and designing and refining the intervention using iterative cycles of development with stakeholder input. The guidance emphasizes the importance of considering each action's relevance to a specific intervention in a specific context, both at the start and throughout the development process. The article also highlights the need for flexibility in applying the guidance, as not all aspects may be relevant to every intervention or context.