Extensively updated in light of the UK Medical Research Council’s (MRC) latest guidance on complex intervention research methods in health and social care, this important book provides the most comprehensive resource available to understand and apply the range of methodologies available to researchers and practitioners today. After an introductory chapter detailing the history of, and future challenges for, complex interventions research, and two chapters on patient and public involvement, the book is split into four discrete but interconnected parts:Section I explores the development phase of complex interventions, covering theory building, adaptation of existing interventions, interest-holder involvement, and context consideration – setting the foundation for successful research programmes. Section II examines feasibility testing of complex interventions, addressing uncertainties in intervention design, methodology, and procedures before determining whether to stop, amend, or proceed to full evaluation. Section III presents evaluation methodologies for complex interventions, covering trial designs, process evaluation, economic assessment, data registries, population approaches, theory-based methods, and simulation modelling. Section IV addresses implementation science methodologies for translating complex interventions into practice, addressing theories, adaptation, hybrid designs, diffusion, and collaborative approaches. The concluding chapter asks readers to reflect on the importance of initiating collaborative, programmatic research groups. With a renewed focus on the context of complex interventions, including concise summaries of the latest thinking, this is essential reading for any researcher or practitioner in health and social care.