The choice of the type of compressor depends primarily on the required flow to be compressed, the density of the gas in conjunction with the total head (for a given gas, this is the compression ratio), and the duty which has to be performed. This chapter covers the minimum requirements, basic reference data, and necessary formulas for process calculations and proper selection of compressors to be used in natural gas processing industries. Compressors are dealt within four groups; axial, centrifugal, reciprocating, and rotary, and each are covered in separate sections.