Gas compressors are widely used to develop higher pressures or reduced volume of gas, needed to transport from the production site to the consumer point. The working principle of compressor is symmetrical to the pump. Generally, compressors either positive displacement - the reduction in volume by a piston or screw element considered in thermodynamics; or centrifugal - imparting kinetic energy to gas by centrifugal force of impeller, are selected depend on end usages. The different types of compressors - screw, reciprocating, centrifugal or some other types are abundantly used for industrial or domestic applications but these machines are least cared in aspects of its condition monitoring or maintenance. Therefore, a theory added practical aspects of compressors in the fields of operation and maintenance are presented in practical and empirical format. The book is divided in six chapters - introduction, Theory of compressors, Reciprocating Compressors, Rotary Compressors, operation of Centrifugal Compressors, and Compressors bearings. Discussions are supported by figures, charts & tables, practical figures, thumb rules, examples, and case studies. The book is presented in hand out form with practical figures and data for easy understanding and guidance to young Engineers in their day to day works.