Microseismic monitoring is a key tool in hydraulic fracturing for shale-gas field development, providing information for various characteristics of induced fractures (e.g., length, height, width, dip, azimuth, asymmetry, and growth rate) and the volume of the stimulated reservoir. The microseismic data are acquired by surface or downhole geophone arrays. Microseismic events are located based on: (1) the arrival times of P-and S-waves and (2) wavefield back-propagation. The main uncertainty in the location of microseismic events results from inadequacy of the velocity model which can be constrained by dipole sonic data, traveltimes from the sources at the known locations, and cross-well tomography.