The availability, resolution, and overall quality of crosswell information can significantly impact reservoir management and optimization, as well as the accuracy of reserves assessment. Standard reservoir mapping methods are limited by a lack of data falling between surface and well-bore measurements. While 3-D seismic interrogates large subsurface volumes, it does not provide sufficient resolution for determining reservoir characterization or for monitoring fluids within the reservoir. Well logs and cores do deliver high resolution and precise quantification; but they provide limited information, given that they sample only a small portion of a very large heterogeneous volume. In addition, data accuracy can be reduced by distortion from rocks and fluids encountered during acquisition. Crosswell seismic imaging is the first technology that fills the information gap between surface seismic and well logs. Applied in a wide range of environments, crosswell seismic enhances the profitable management of subsurface hydrocarbon reserves. The effective management of subsurface resources requires a true model by which a reservoir's production performance can be predicted -- a model that not only quantifies the reservoir's properties but depicts their distribution as well. Crosswell seismic meets these requirements. With crosswell seismic, critical reservoir characteristics are directly imaged, not statistically estimated. Faults and stratigraphic features can be clearly identified prior to drilling. Trajectories for in-fill and horizontal wells can be precisely plotted. Fluid movements and saturation changes can be monitored in time-lapse mode. These and other capabilities combine to make crosswell seismic a highly effective tool for managing both resources and risks.