Situation Calculus Meets Modal Logic Johan van Benthem Abstract: Situation Calculus and Modal Logic sometimes seem 'worlds apart', but appearances are deceptive. Taking Reiter's book "Knowledge in Action" as our running reference, we show how the two research agendas are closely related. Circumscription and varieties of non-monotonic logic can be seen as variants of modal logics of belief revision, and key SitCalc computational methods like regression are equivalent to techniques involving 'reduction axioms' in dynamic-epistemic and in epistemic-temporal logic. We also discuss how ideas from Situation Calculus could still beneficially influence modal logic.