With high-permeability hydrocarbon reservoirs exhausting their potential, developing low-permeability reservoirs is becoming of increasing importance. In order to be produced economically, these reservoirs need to be stimulated to increase their permeability. Hydraulic fracturing is a technique used to do this. A mixture of water, additives, and proppants is injected under high pressure into the subsurface; this fluid fractures the rock, creating additional pathways for the oil or gas. Understanding the nature of the resulting fracture system, including the geometry, size, and orientation of individual fractures, as well as the distance from one fracture to the next, is key to answering important practical questions such as: What is the affected reservoir volume? Where should we fracture next? What are the optimal locations for future production wells?