This core textbook introduces the key concepts, theories, models and frameworks used in implementation science, and supports readers applying them in research projects. The first part of the book focuses on the theory of implementation science, providing a discussion of its emergence from the evidence-based practice movement and its connections to related topics such as innovation research. It includes chapters looking at a wide range of theories, methods and frameworks currently used in implementation science, and a chapter focusing on suitable theories that could be imported from other fields. The first part also addresses strategies and outcomes of implementation and discusses how researchers can build causal pathways adapted to their study. The second part of the book focuses squarely on putting the theory of implementation science to work in practice, with chapters discussing research methods used in the field and how to select the most appropriate approach. This section also features several chapters presenting in-depth case studies of specific applications. This multidisciplinary text is an essential resource for graduate students from a range of healthcare backgrounds taking courses on implementation science, as well as researchers from medicine, nursing, public health, allied health, economics, political science, sociology and engineering. © 2024 selection and editorial matter, Per Nilsen; individual chapters, the contributors.