Process simulation is an important activity in process engineering covering the whole life cycle of a process, from research and development to conceptual design and plant operation. In this context, flowsheeting is a systemic description of material and energy streams in a process plant by means of computer simulation with the scope of designing the plant or understanding its operation. Steady-state flowsheeting is an everyday tool of the chemical engineer. This chapter provides an overview of computer simulation in process engineering, including the key steps in a simulation approach, the architecture of flowsheeting software and the integration of simulation tools. Particular emphasis is set on the necessity of analysing the problem before rushing to computer simulation.