Background: Researchers publish the processes they use to develop interventions to improve health. Reflecting on this endeavour may help future developers to improve their practice. Methods: Our aim was to collate, describe, and analyse the actions developers take when developing complex interventions to improve health. We carried out a systematic mapping review of empirical research studies that report the development of complex interventions to improve health. A search was undertaken of five databases over 2015-2016 using the term 'intervention dev*'. Eighty-seven journal articles reporting the process of intervention development were identified. A purposive subset of 30 articles, using a range of published approaches to developing interventions, was selected for in-depth analysis using principles of realist synthesis to identify the actions of intervention development and rationales underpinning those actions. Results: The 87 articles were from the USA (39/87), the UK (32/87), continental Europe (6/87), and the rest of the world (10/87). These mainly took a pragmatic self-selected approach (n = 43); a theory-and evidence-based approach, e.g. Intervention Mapping, Behaviour Change Wheel (n = 22); or a partnership approach, e.g. community-based participatory research, co-design (n = 10). Ten actions of intervention development were identified from the subset of 30 articles, including identifying a need for an intervention, selecting the intervention development approach to follow, considering the needs of the target population, reviewing published evidence, involving stakeholders, drawing or generating theory, and designing and refining the intervention. Rationales for these actions were that they would produce more engaging, acceptable, feasible, and effective interventions. Conclusions: Developers take a variety of approaches to the international endeavour of complex intervention development. We have identified and described a set of actions taken within this endeavour regardless of whether developers follow a published approach or not. Future developers can use these actions and the rationales that underpin them to help them make decisions about the process of intervention development.