A simulation is an orchestrated series of operations used to model the behaviour of a system. The system being simulated could exist, such as the world economy, have existed, such as a transition in evolutionary history, or be hypothetical, such as the synthetic world of a computer game. A simulation will typically model how a system develops over time, and potentially at a fidelity that is hard or impossible to achieve using other kinds of model. Most simulations are specified formally, either mathematically or algorithmically, and executed on digital computers. As a tool for modelling and understanding the natural world, simulation is used throughout the sciences to model systems ranging from the global climate, financial markets, immune systems, power plants, and the origins of language. Simulation made possible, and is the primary method of inquiry for, fields such as artificial intelligence and complexity science. More broadly, simulation is used to engineer virtual environments for training and entertainment, create artefacts, and explore design spaces. High fidelity simulations that operate in real time can blur the distinction between virtual and natural worlds. In each of these applications, simulation provides a way to create, analyse, manipulate, and crucially, automate, models of the possible with an arguably unparalleled flexibility and scope.