As introduced in Chapter 1, production rates from unconventional wells decline very rapidly during the first 2–3 years of production. In this chapter we first demonstrate that production rates (and cumulative production) are dominated by linear flow from the almost impermeable matrix into much more permeable fracture planes. The permeable fracture planes consist of the hydraulic fractures themselves and the pre-existing fractures and faults that have slipped in shear during stimulation. As we show, the rapid decrease in production rates is a natural consequence of depletion in these extremely low permeability formations. We argue that the cumulative area of permeable fracture planes created during stimulation is a key factor influencing ultimate resource recovery.