Here, we explain why off-design performance calculations require an iterative approach to maintain continuity in mass and energy flows in component matching. We start with a simple example of a turboshaft engine with a free power turbine and show how the number of iteration variables increases as the engine architecture becomes more complex. We demonstrate how the use of the Newton–Raphson algorithm may be extended to any number of variables as well as dynamic engine simulations. We conclude the chapter with some examples of problems that may occur when setting up iteration schemes and give practical advice on how to resolve them.