This chapter provides a comprehensive overview of rotor design principles for centrifugal compressorsCentrifugal compressor. The rotating assembly comprises several key components, including the shaft, impellers, sleeves, balance piston(s), thrust disk, and coupling hubsHub. Shaft design is guided by both lateral and torsional rotordynamic behavior, as well as its role in transmitting torque to the impellers and any additional casings located at the nondrive end. Material selection for the rotor assembly is influenced by factors such as process gas composition, operating and design parameters, and the installation environment. This chapter outlines a systematic approach to rotor design, examining how shaft length and diameter affect rotordynamic performance, and comparing stiff versus flexible shaft configurationsFlexible shaft design along with their respective applications. It also covers shaft manufacturing techniques and commonly used materials. Additional topics explored in detail include impeller thrustImpellerthrust forces, sizing criteria for balance pistonsBalance piston sizing, the types of interstage and balance piston seals, interference fitsInterference fits on the shaft, and protective surface treatments for rotor components.