Emission standards derived to protect the environment have driven engine manufacturers to accelerate their efforts in engine emission development in order to meet the mandated standards. This paper will outline the current and upcoming emission standards, the technology involved in accomplishing the requirements and the solutions to in-service engines meeting their emission standards. The paper targets the fleet operator to provide a single source of information in an effort to alert them to upcoming requirements and what can be expected when specifying equipment for procurement. The technology discussed in this paper varies in its application for several engine makes and models. Since technology transfers from application to application it is vital that these ideas and concepts be explained.