Implementation science has seen the development of literally hundreds of theoretical approaches in the form of theories, models and frameworks for use by researchers in the field. This chapter provides an overview of the theories, models and frameworks that are available in implementation science, with the aim of describing how these approaches are applied. The chapter presents a taxonomy that distinguishes between different approaches to advance clarity and achieve a common terminology in the field. Knowledge about the theories, models and frameworks facilitates appropriate selection and application of relevant approaches in implementation science studies. © 2024 selection and editorial matter, Per Nilsen; individual chapters, the contributors.