Couplings are essential components in machinery trains, yet they are often overlooked during the specification process by engineering contractors and owner companies. In many cases, users base their selection solely on basic parameters such as operating speed, torque, safety factor, and shaft dimensions. However, a comprehensive evaluation should also include additional critical factors—such as combined steady-state and cyclic stresses, angular misalignment tolerance, torsional rigidity, axial displacement capacity, axial natural frequencies, anticipated peak and transient torques, and both lateralLateral frequency and torsional critical speedsCritical speed. To ensure a well-informed selection, this evaluation should be a collaborative effort involving the primary equipment vendor, subvendors for the driver and gearGear units (if applicable), the owner’s machinery engineers, and the engineering contractor. Choosing the appropriate coupling is a key contributor to the long-term reliability and operational availability of machinery trains in process plants. This chapter highlights two of the most commonly used high-performance, non-lubricated coupling types in modern centrifugal compressorCentrifugal compressor trains: metal diaphragm couplings and disk-pack couplings. It also covers standard selection methodologies, typical construction materials, balancingBalancing practices, torque monitoring strategies, and coupling guard design considerations.