Understanding the flow and sorption properties of unconventional reservoir rocks is essential for predicting the movement of water and hydrocarbons during stimulation (injection), production and depletion. Unconventional reservoir rocks are unified (and essentially defined) by their ultra-low matrix permeability, which is approximately a million to a billion times less than that of conventional reservoir rocks. For this reason, hydraulic fracturing and shear stimulation on pre-existing faults (Chapters 8 and 10) are necessary to expose more surface area of the matrix for production. Although the initial flow behavior during injection and production is controlled by the properties of the fracture network, the long-term flow behavior is controlled by the ultra-low permeability matrix.