Rockfalls pose a significant threat to life and property, although significant advances in rockfall protection have been made in the past decade. Determining rockfall processes and related hazard, however, remains a difficult task because of the complexity and intrinsic stochastic nature of the physics involved. The appropriate application of rockfall modeling tools requires a thorough understanding of their logic, assumptions, advantages, and limitations, as well as careful assessment of rockfall sources, block and slope characteristics, and model calibration data. This chapter provides a discussion of major issues in rockfall definition, characterization, and modeling, with special emphasis on rockfall runout. Our discussion is supported by modeling examples carried out using the 3D simulator Hy-STONE. Different modeling approaches are critically evaluated, including the empirical shadow angle method, and 2D and 3D mathematical models. Application of the shadow angle concept requires the user to be aware of several issues related to definition of the shadow angle and the effects of morphological constraints. Most limitations of empirical approaches can be overcome with mathematical models that account for slope morphology and roughness, energy dissipation at impact or by rolling, and the effects of vegetation, block fragmentation, and block–structure interaction. We discuss different modeling approaches and calibration problems and the important dependency of model parameters and results on correct characterization of the topography.