Compressors play important role in industries such as petroleum, oil and gas and … as the heart of the plant. These machines are driven by electric motors or turbines either steam or gas. Compressors are used for compressing the gases fluids. Compressors which are used for compressing fluids are knows as pumps. In some equipment such as gas turbines, compressor and turbine are mounted on a single rotor and after compressing the combustion air by compressor, it is sent to combustion chamber. Compressors are categorized in two main groups; dynamic & displacement. In displacement compressors (such as piston compressors) the pressure of the gas is increased by decreasing its volume. Centrifugal compressors and axial flow compressors are of the dynamic compressors category. Displacement compressors are divided into two main categories; reciprocating compressors & rotating compressors. Knowing the fundamentals, applications, maintenance and design requirements of different compressors is of the essence for those who are involved in this field. Whole of these topics will be discussed in compressor training course.