Fluid-filled fractures and microcracks are the most stress sensitive component of crustal rock and often determine the pathways and volume of crustal fluid movement. If we are to comprehend the role of fractures and fluids in tectonic processes, to formulate an accurate hypothesis for phenomena precursory to catastrophic seismic failure, or to monitor hydrocarbon and geothermal reservoirs for the presence or absence of major fluid pathways, we must understand how seismic waves interact with the fracture, crack, and microcrack structures within the rock mass. A salient feature of fracture-related rock structure is the effective seismic anisotropy of aligned fractures and microcracks. With the advent of large-volume digital seismic data sets it is feasible to use polarized shear waves to explore the anisotropic fracture properties of the crust. Crustal fracture structures which may be monitored by polarized shear waves range in dimensions from the 10--100 km scale of crustal stress orientation and preseismic stress buildup, through the 100-1000 m scale of fluid reservoir structure and characterization, and the 1-10 scale of mining and geoengineering rock strain monitoring, to millimeter and micrometer or possibly smaller fluid-filled inclusions determining the elastic response of the intact rock mass. To focus developments in seismic anisotropic studies of crustal fracture phenomena, the American Geophysical Union and the Society of Exploration Geophysicists sponsored a Chapman Conference on Seismic Anisotropy in the Earth's Crust, held May 31 to June 4, 1988, at the Lawrence-Berkeley Laboratory of the University of California. This was the Third International Workshop on Seismic Anisotropy (3IWSA). Nineteen conference papers are presented in this special section of the Journal of Geophysical Research. ¿ American Geophysical Union 1990