Inlet flow distortion is defined in simple models along with its effect on engine stability and performance. We explain why circumferential distortion is more important than radial distortion and justify the suitability of the pressure distortion coefficient, DC60, as a descriptor. This is followed by a worked example of parallel compressor theory in GasTurb, which is a simple method to quantify the change in surge margin with varying degrees of distortion. The theory is then extended to cover a multi-spool compression system. The effect of aerodynamic coupling between sequential compressors connected by short ducts with many struts is addressed in detail. The chapter concludes with a discussion on the response of the engine control system to distortion, including the sensitivity to the choice of the thrust (power) setting parameter.