After a review of the various cryogenic cycles for the liquefaction of natural gas (LNG), the layout of turbo-compressors for large liquefaction plants is explained. Formulae are given for the rapid determination of the principal dimensions, permitting an easy comparison of the relative merits of radial and axial machines. The solution of the problem concerning the adaptation of the axial compressor to various working conditions, such as those prevailing during starting by using different gases, is discussed.