Modern gasoline engines have to meet increased stringent emission requirements along with the demand of a better fuel economy. This has led to a transition from carburetor to port fuel injection (PFI) mode in developing world for engines comprising mostly of two- and three-wheeler segments and small-scale power generation sets. Therefore, a thorough understanding on mixture formation and combustion phenomenon is needed to further enhance PFI engines. Planar laser-induced fluorescence (PLIF) has proved to be a successful optical diagnostic technique that can provide very high spatial resolution images of fuel distribution in the region of interest. This has led to a direct visualization of fuel distribution with evaluation of both spatial and temporal variations. It has furthered understanding of various engines parameters that affect mixture formation process. Various exciting concepts about fuel stratification have been proposed over the years for enhanced engine operations at lean equivalence ratios. These have been verified and optimized by information gathered from PLIF. In this review article, the authors explain mixture formation process right from the point of fuel injection in intake manifold, the subsequent formation of fuel films and its impact on engine operation. Several PLIF studies on fuel distribution, its spatial and cycle-to-cycle inhomogeneities, effects of injection timing, flow field, equivalence ratio and engine speed on mixture formation have been discussed in separate subsections. Furthermore, studies involving concepts of fuel stratification have also been briefly discussed.