Injection molding is the most important cyclic polymer molding process, while mold represents the main part of the system. Creation and final properties of the part are influenced by numerous parameters, and mold cooling is one of the most important, achieved by mold cooling system. It consist of a series of cooling channels, placed near the mold cavity, through which the cooling fluid flows. By designing an optimal cooling system, it is possible to achieve the increased productivity and economy, as well as improved quality of the part. In this paper are described and elaborated cooling mold and system cooling mold procedures. A brief overview of heat transfer and exchange modes within the mold is given, and the parameters for selecting and creating cooling channels are listed. The mold cooling circuits are described and shown in detail, from basic ones to the complex ones, which are used in complex parts with high quality requirements. Cooling is acomplished by various devices and cooling fluids, which are also described. Finally, an analysis for designing an optimal mold cooling system is implemented.