An optically accessible, DI Diesel engine was used to investigate the effect of swirl on fuel-air mixing and flame evolution. Quiescent and swirling conditions were studied at three different fuel-air ratios at an engine speed of 900 RPM. For the mixing studies, performed with nitrogen to prevent combustion, a mirrored piston was used to permit double pass shadowgraph imaging within the combustion chamber. High speed shadowgraph cinematography, using an Argon ion laser, yielded insight into the temporal evolution of the fuel jet and permitted the calculation of penetration speeds and area of the fuel jet as a function of time. With swirl, the penetration rate of the fuel jets was reduced, and the area of the over which fuel was observed increased by 25 percent. Combustion phenomena were studied using backlighting so that the spray and visible light from combustion could be recorded on high speed video. For the combustion studies, the engine was configured to permit the entire combustion volume, including the squish region, to be observed. The evolution of the fuel jets and the visible flame in the combustion chamber were significantly altered by swirl in manners which were consistent with the mixing effects observed in the shadowgraph study. With swirl, the emergence of the visible flame into the squish region was slower and its maximum radius was reduced by approximately fifteen percent compared to the quiescent condition at the same fuel-air ratio.