In many unconventional shale reservoirs, preexisting natural fractures play a critical role in the creation of complex hydraulic fracture networks during stimulation due to the interaction between hydraulic fractures and the natural fractures or activation of the natural fractures by the fracture stimulation to provide enhanced permeability to increase the hydrocarbon production. In this chapter, we discuss the mechanics influencing the crossing behaviors when a hydraulic fracture intersects a natural fracture, which is a critical process influencing the generation of complex fractures. A complex hydraulic fracture network model that incorporates mechanical interaction between hydraulic fracture and natural fracture, as well as among hydraulic fractures, will be presented. We will then present examples of fracture simulations for various configurations of the natural fractures, in terms of some key parameters, such as their orientation, density, and length, to highlight the impact of the natural fractures on the generated hydraulic fracture network geometry. The impact of the resulting geometry on proppant distribution in the fracture network and production will also be examined.