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An introduction to the operational and design features of the transonic fan stage of a modern turbofan engine can give the student an example of highly developed turbomachinery whose performance and design can be readily appreciated from fundamental knowledge of compressible fluid flow and boundary layer separation. Application of continuity and momentum principles along with the constraint of a limiting tip-diameter Mach number (relative to the blade) can serve to determine the required fan size, RPM, blade angles and blade spacing for a given mass flow rate and inlet stagnation pressure and temperature. Sensitivity of fan design parameters to limiting Mach number, hub-tip ratio and stall safety margin is readily determined by a simplified model which is compatible with the results of detailed design procedures.