Implementation science researchers have a multitude of theories, models and frameworks (TMFs) at their disposal. These TMFs provide a rich source of potentially inspiring ways to plan, conduct and evaluate implementation projects. They can enhance researchers’ ability to generalize findings, promote shared understanding and advance the field. However, despite their many strengths, TMFs tend to be underused, used superficially or misused. There is also a tendency to use convenient or familiar TMFs. This chapter addresses the challenges of identifying and using appropriate TMFs and describes the T-CaST (Theory Comparison and Selection Tool), a tool that was developed to facilitate the selection of TMFs. © 2024 selection and editorial matter, Per Nilsen; individual chapters, the contributors.