In recent years the automotive industry has been forced to reduce the harmful and pollutant emissions emitted by direct-injected diesel engines. To accomplish this difficult task various solutions have been proposed. One of these proposed solutions is the usage of gaseous fuels in addition to the use of liquid diesel. These gaseous fuels have more gasoline-like properties, such as high octane numbers, and thereby are resistant against auto-ignition. Diesel on the other hand, has a high cetane number which makes it prone to auto-ignition. In this case the gaseous fuel is injected in the inlet manifold, and the diesel is direct injected in the cylinder at the end of the compression stroke. Thereby the diesel fuel spontaneously ignites and acts as an ignition source. The main goals for the use of a dual-fuel operation with diesel and gaseous fuels are the reduction of particulate matter (PM) and nitrogen oxides (NOx) emission. Furthermore, the application of such a dual-fuel operation can offer potential economic and efficiency advantages. Depending on the gaseous fuel used, these goals can be achieved. In general, dual-fuel combustion of gaseous fuels and diesel decreases soot emissions compared with normal diesel combustion except for syngas. Furthermore, increasing load and/or gaseous fuel content leads to a further decrease in soot emissions. Both the application natural gas and liquefied petroleum gas as gaseous fuel offer the possibility to diminish nitrogen oxide emissions probably due to homogenous mixture compositions and/or decreased mixture temperatures. However, using hydrogen or syngas in dual-fuel combustion tends to increase nitrogen oxide emissions; this might be due to the higher flame temperatures and combustion rates of these gasses. Furthermore, the emissions of unburned hydrocarbons and carbon monoxides tend to increase for all evaluated gaseous fuels with dual fuel combustion mainly due to incomplete combustion of mixture trapped in crevices. Efficiencies of the different gaseous fuels are in the same order of magnitude. Some seem to lead to slight efficiency improvements (hydrogen and LPG) while others result in a slight decrease (natural gas and syngas). However, the significant price difference of natural gas and LPG compared to diesel can offer a considerable economic advantage.