The conventional concept of hydraulic fracturing is that a single planar fracture forms at the wellbore and propagates through the formation. In recent years, there has been a growing realization that stimulation often generates fracture networks that are volumetric and complex. Network complexity is valuable for enhanced recovery because it increases stimulated fracture surface area. The complex fracture network concept has the potential to impact stimulation design and modeling, formation evaluation from well logs and other data, measurements of the minimum principal stress, and many other areas. However, before these concepts can be applied fully, it will be necessary to resolve fundamental questions about the physical processes that give rise to complexity and to develop new modeling tools that are capable of describing these processes. In this article, I review the literature to describe a few conceptual models that have been proposed to explain complexity in unconventional resources. I discuss the need for diagnostic techniques to distinguish among these mechanisms, and I provide a few examples of such techniques. Finally, I review the current state of the art for modeling complex hydraulic fracturing and give an example using Complex Fracturing ReAsearch Code (CFRAC), a recently developed hydraulic-fracturing simulator. The simulation results demonstrate that the stresses induced by fracture deformation have a first-order impact on how stimulated fracture networks develop.