Using natural gas as a fuel in internal combustion engines is a promising way to obtain efficient power generation with relatively low environmental impact. Dual fuel operation is especially interesting because it can combine the safety and reliability of the basic diesel concept with fuel flexibility. To deal with the greater number of degrees of freedom caused by the interaction of two fuels and combining different combustion regimes, it is imperative to use simulation methods in the development process to gain a better understanding of the combustion behavior. This paper presents current research into ignition and combustion of a premixed natural gas/air charge with a diesel pilot spray in a large bore diesel ignited gas engine with a focus on 3D-CFD simulation. Special attention was paid to injection and combustion. The highly transient behavior of the diesel injector especially at small injection quantities poses challenges to the numerical simulation of the spray. Design of Experiments (DoE) methods were applied to identify adequate parameter sets for the spray models. Dual fuel combustion is depicted with a widely used approach, the Extended Coherent Flame Model with 3 zones (ECFM-3Z), with which it is possible to calculate all three combustion regimes simultaneously. Several adjustments are necessary to depict the dual fuel combustion processes accurately, namely the treatment of ignition delay for the dual fuel mixture, the initial flame surface density and the flame front propagation throughout the lean gas-air mixture. Detailed chemistry calculations using a dual fuel compatible reaction mechanism have been performed for the ignition delay tabulation, which has been extended to cover the two different fuels used in this combustion mode. A formula for the initial flame surface density that includes thermal expansion of the gas and turbulence influences has been derived. The flame front propagation is then also influenced by the laminar flame speed, a parameter that depends on the fuels. Finally, these models are validated with measurement data from a single cylinder research engine.