Hydraulic fracturing (hydraulic fracture stimulation, fraccing) is a process through which a large number of fractures are created mechanically in the rock, thus allowing the natural gas and/or crude oil trapped in subsurface formations to move through those fractures to the wellbore from where it can then flow to the surface. Hydraulic fracturing can both increase production rates and increase the total amount of gas that can be recovered from a given volume of shale. Pump pressure causes the rock to fracture, and water carries sand (“proppant”) into the hydraulic fracture to prop it open allowing the flow of gas. While water and sand are the main components of hydraulic fracture fluid, chemical additives are often added in small concentrations to improve fracturing performance. It is the purpose of this chapter to present the details of hydraulic fracturing and the means by which the process is applied to tight shale formation and other tight formations. The chapter stresses the importance of understanding where hydraulic fracturing fits into the entire drilling, well construction/completion and production phases of natural gas and crude oil recovery activities.