The central question of Simulation Theory is: Are we living in a simulation? Before that question can be answered, we need to know what “simulation” means, and, it turns out, there are a finite set of possible ways to understand the meaning of “simulation.” This book systematically works through the possible meanings of simulation, addressing the central question of Simulation Theory for each understanding. While providing an essential overview, this book reveals the systematic internal organization of Simulation Theory. Next-level insights become available by way of this information, rendering readers able to think through the question, “Are we living in a simulation?” and to discuss Simulation Theory more coherently and at a greater depth. This book addresses both the Computer Science and the Philosophy of Simulation Theory. Perennial topics such as The Matrix films, A Glitch in the Matrix documentary, the video game Simulation Hypothesis, Philip K. Dick’s Metz lecture, Nick Bostrom’s Simulation Argument, and insights gained by philosophers from Plato to Deleuze and Baudrillard.