Compressor maps, physically sound over the complete range of operation, are critical to accurate aircraft engine performance simulations. Calculated and measured maps usually cover only the speed range between idle and full thrust. Simulation of cranking, starting, windmilling and re-light requires maps which include extreme sub-idle speed values as well as consideration of the map region where the pressure ratio is less than unity. Engineers outside industry, at universities and research facilities, usually lack access to the special rigs needed for measuring phenomena such as locked rotor (zero-speed line) characteristics, nor do they have the geometric data needed for CFD calculations. In this chapter, a map extension method is described which employs interpolation between the idle- and zero-speed line that needs neither data from an experiment with a locked rotor nor compressor geometry information. Zero-speed torque and pressure losses are derived from correlations embedded in the original non-extended map. A detailed description of the associated flow phenomena is also provided, and compressibility effects are addressed.