Principles of gas turbine off-design behavior is discussed in detail. Understanding the interactions between individual components is a prerequisite of the effective and professional use of any gas turbine performance program. We start with the simplest configuration of a single-spool turbojet and study the operating lines in the compressor and turbine maps. We use mainly non-dimensionals to explain the relationships between the components, including the lack of influence of variable guide vanes on a compressor operating line. The discussion is then extended to cover two-spool turbojets and turbofans. We describe why the operating line of the fan is dependent on the bypass nozzle flow characteristic and pay special attention to why subsonic and transonic boosters behave differently at part power. We also explain why the operating line of an LP turbine is determined by the core nozzle pressure ratio and hence varies with bypass ratio. Finally, we look at the behavior of the components in both single- and multi-spool turboshafts with constant speed power turbines. The validity of all the results is illustrated extensively by calculations from GasTurb.