Concerns about health and global warming have influenced the application of stricter regulations and norms, related to the emission of harmful gases, in the automotive industry, with the aim of introducing "cleaner" vehicles. Using an exhaust aftertreatment devices, for the purpose of reducing NOx emissions, is quite expensive, so implementing an EGR that contributes to reducing NOx, in a cylinder, during the combustion process is one of the better options. Simple pneumatic EGR valves have evolved over time into electronically controlled and regulated EGR modules, which have the ability to cool exhaust gases. Part of the exhaust gases from the engine is recirculated through the EGR system, back into the cylinders, thus reducing the combustion temperature and the formation of nitrogen oxides. The goal of this paper is to minutely analyze the construction of the EGR system in Otto and Diesel engines. The design of the internal exhaust gas recirculation system and the external exhaust gas recirculation system directly affects the cooling of the exhaust gases. The main division relates to high pressure and low pressure EGR system. Different types of EGR valves, EGR valves with regard to the control mode that can be controlled by exhaust overpressure and underpressure, as well as a digital electronically controlled EGR valves will be analyzed. Also, the paper will explain the types of exhaust gases in Otto and Diesel engines, harmful and harmless emissions, and their impact on people and the environment. The last chapter is intended to analyze the principles of operation of the EGR system in Otto and Diesel engines, its impact on reducing the concentration of exhaust gases, and the impact on the performance of the engine.