Abstract: Impact cratering is a fundamental geological process that shapes the surface morphology of planets and small bodies. The morphological characteristics of impact craters and the distribution of ejecta record key information about impact conditions and target properties. Classical impact-cratering theories are mainly based on idealized assumptions, such as vertical impact, homogeneous targets, and flat surfaces. However, natural impact events are generally oblique and commonly occur on targets with complex topography and heterogeneous structures. Impact angle, topographic features, and target heterogeneity jointly affect shock-wave propagation, energy deposition, material failure, ejecta motion, and crater collapse, resulting in pronounced asymmetry and complex structural characteristics of natural impact craters. This paper reviews recent progress in the formation mechanisms of impact craters under complex impact conditions, with emphasis on the effects of oblique impact, topography, porous media, layered structures, and joints and fractures on the cratering process, ejecta distribution, and scaling-law models. First, impact-cratering experiments, numerical simulation methods, and classical scaling laws are summarized. Then, the basic theories of ejecta motion under vertical and oblique impact conditions are introduced. The controlling effects of impact angle on the shock-pressure field, transient-crater evolution, final crater morphology, and scaling laws are analyzed. Furthermore, changes in shock-wave attenuation, energy dissipation, and material deformation mechanisms under complex target conditions are discussed. Existing studies still face several challenges, including insufficient experiments on multi-factor coupling, lack of unified characterization of complex target parameters, difficulties in scaling among experimental, numerical, and natural impact conditions, and unclear applicability limits of current scaling laws. Future research should focus on systematic experiments under coupled oblique-impact and complex-target conditions, high-resolution three-dimensional numerical simulations, and planetary observations, so as to develop cratering theories and quantitative prediction models that are more applicable to natural impact scenarios.