As low permeability reservoir development matures in the US, operators are now contending with the issue of field development. Specifically, optimal well spacing and timing of infill drilling must be addressed. This is largely driven by the matrix perm and extent of the effective hydraulic fracture area. The structural setting largely dictates the dimensions of the hydraulic fracture system, both in height growth and lateral complexity. But depletion from parent wells can impact the dimensions of both the created frac system as well as the productive frac system on infill wells. Thus geologic structure and depletion need to be taken into account to determine not only where to place infill wells, but as importantly when to place infill wells. Examples from the Bakken and Woodford Shales will be shown to demonstrate the workflow required to answer these field development questions.