Modality, Bisimulation and Interpolation in Infinitary Logic Johan van Benthem Abstract: Rohit Parikh has been one of the pioneers at the interface of modal and dynamic logic. Bringing dynamic, process­oriented concerns into modal logic has been a major move, whose repercussions are still being felt today. In this contribution, I consider the bridge concept of bisimulation, which links modal logic with computational process theories. Our main results show how its model­theoretic invariance properties, first established within first­order model theory, may be lifted to infinitary logic, a natural generalized habitat for theories of programs and processes. The techniques used for this purpose also suggest some further proof­theoretic uses, going beyond purely modal languages.