Many of today’s software systems are parallel or concurrent. With the rise of Node.js and more generally event-loop architectures, many systems need to handle concurrency. However, their non-deterministic behavior makes it hard to debug. Today’s interactive debuggers do unfortunately not support developers in debugging non-deterministic issues. They only allow exploring a single execution path. Therefore, some bugs may never be reproduced in the debugging session, because the conditions to trigger are not reached. As a solution, we propose multiverse debugging, a new approach for debugging non-deterministic programs that allow developers to observe all possible execution paths of a parallel program and debug it interactively. We introduce the concepts of multiverse breakpoints and stepping, which can halt a program in different execution paths, i.e. universes. We apply multiverse debugging to AmbientTalk, an actor-based language, resulting in Voyager, a proof of concept multiverse debugger that takes as input Featherweight AmbientTalk programs written in PLT-Redex, and allows programmers to interactively browse all possible execution states by means of multiverse breakpoints and stepping commands. We provide a proof of non-interference, i.e we prove that observing the behavior of a program by the debugger does not affect the behavior of that program and vice versa. Multiverse debugging establishes the foundation for debugging non-deterministic programs interactively, which we believe can aid the development of parallel and concurrent systems.