Theories have been developed by researchers in implementation science to achieve enhanced understanding and explanation of how or why implementation succeeds or fails. These theories specify the causal mechanisms of change. This chapter looks at the concept of theory and examines how implementation science-specific theories are defined. The chapter also clarifies how theories used in implementation science can be distinguished from models and frameworks in the field. Three implementation theories that are used in implementation science are described: Normalization Process Theory (NPT), Organizational Readiness for Change (ORC) and Capability Opportunity Motivation - Behaviour (COM-B). © 2024 selection and editorial matter, Per Nilsen; individual chapters, the contributors.