Many assumptions are made when scientists build computer simulations. For some philosophers of science, these assumptions present a novel epistemological problem. Much of the philosophical work done on computer simulation has taken place in isolation to recent work done on scientific models. In this paper, I outline a conceptual framework of scientific models and apply this framework to computer simulations to make sense of the supposed novel epistemological problem. I then show how analogous problems take place in the context of scientific modelling more generally.