The selection and procurement of compressors and steam turbines for use in the chemical and process industry is highly interdisciplinary. The success of a project is determined by a number of areas of knowledge: from mechanical, electrical, materials and control engineering knowledge to thermodynamics, fluid mechanics and strength theory through to project management and quality control. In this guide, the individual steps are presented along the chronological chain, together with the basic decisions and pitfalls that need to be taken into account. The work is limited to custom-built machines that are specially optimized for a specific process and to gases and vapours as conveying media. It is presented from the operator's point of view with a focus on high system availability, safety and favorable conditions for maintenance and servicing.