The dual-fuel diesel/natural gas (NG) concept is based on the introduction of a homogeneous charge of air and NG in a diesel engine. The charge is then ignited by the combustion of a small amount of diesel fuel directly injected into the cylinder near the top dead center. Such technology can be considered as a promising and inviting solution to address the tradeoff of particulate matter-NOx emissions affecting traditional diesel engines. In this paper the conversion of a heavy-duty diesel engine to dual-fuel operations is discussed. Experimental tests were performed to define engine performance and reduce exhaust emissions. The experimental data also were used to develop a numerical analysis, characterized by a mixed one-dimensional (ID)-three-dimensional (3D) approach. As far as 3D simulation tools are concerned, a modified version of the KIVA-3V code was used to simulate the whole working cycle of the engine and to represent diesel injection and overall combustion.