Computers are simulators and computational technologies are simulation technologies. These claims constitute my primary thesis; they specify necessary but not sufficient conditions for computers and computation. I argue for this thesis in two parts: articulation and defense of an account of simulation; and articulation and defense of an account of computation on its basis. In dialogue with prior work, I argue that all simulations have six features: they are representational, interpreted, dynamic, interactional, virtual, and subsistent. Whatever else computers and computation are, they must at least be simulators by the above definition; else they would lack the semantic significance and user manipulability that are signatures of computation and computational processes. I further argue for this thesis “empirically” through consideration of computational architectures that might be thought non-simulational such as: (a) Babbage’s Difference Engine and Analytical Engine; (b) Turing machines (“Turing computability”); (c) electronic calculators; (d) robotics; and (e) humans considered as computers (e.g. the human profession of “computer” in the 1930s-50s; the computational theory of mind). In a third and supplementary part, I discuss what would need to be added to simulation to produce a sufficient definition of computation, as well as some implications of the main thesis if true.