Modifications to refinery processes can control the physical properties of a fuel, how it combusts, and therefore to some degree the emissions from the internal combustion engines. However, some of a fuel's desired properties cannot be delivered through modification to these refining processes. In many of these cases, additives can be used to provide these additional desired performance features. Federal regulations require control of emissions from gasoline engines. This has led to significant changes to engines, electronic controls, and after treatment systems to improve fuel efficiency while maintaining acceptable emission levels. Modern fuels must also change to deliver the required performance in these new engines. Fuel cleanliness and stability become more critical in these more sophisticated engines, and the fuel delivery system must be kept clean to maintain optimum fuel spray development and delivery. Additives play an important role in delivering these enhanced fuel properties. Diesel fuel additives are a necessity today, especially since the advent of, ultra-low sulfur diesel fue, which was created to achieve fuel and air quality regulations and enable after treatment devices. There are several properties, such as lubricity, that became less acceptable as a result of the hydrotreating process required to remove fuel sulfur. Other recent developments such as the inclusion of biofuel blend components in diesel fuel and the introduction of high-pressure common rail fuel injection systems in diesel engines have created a need for additional diesel fuel additives to make the fuel fit for purpose. The major functions of diesel fuel additives are improving those properties that are unacceptable, or enhancing engine performance.