Fundamentals of transient engine performance are explained. Starting with the simplest approach of considering only moments of inertia, we compare two main options for fuel scheduling (Ndot and WF/(N P3) and their effects on the compressor operating line. We then extend the simulation to include mechanical and thermal aspects and illustrate why consideration of compressor and turbine tip clearances is essential for a realistic assessment of transient behavior and operability. We show that, by using the data available from the conceptual design feature in GasTurb, it is possible to generate a simple but realistic simulation of the effects of tip clearance in a turbofan, which can easily be calibrated to real engine measurements or analytical data. Finally, we illustrate the benefits of considering tip clearance with examples of two engine transients, acceleration of a cold engine and a hot re-slam, where a deceleration is followed by an acceleration.