Induced seismicity has been observed near Monticello Reservoir, South Carolina, since December 1977. Deployment of a seismic network before impoundment allowed for detection and accurate location of pursuant seismicity since its inception. Corroborative fault-plane solutions, together with geological and borehole data on fracture orientations, made it possible to determine the structures associated with the initial seismicity. Earlier descriptions attributed this seismicity to the undrained elastic response to impoundment of the reservoir or to a coupled poroelastic response, where diffusion of pore pressure and subsequent weakening was the predominant cause. Quantitative evaluation of strength changes at hypocentral locations of a subset of 53 well-located earthquakes that followed the initial impoundment led to the following results: (1) The rocks in the vicinity of Monticello Reservoir are critically stressed, and strength changes less than or equal to 0.1 MPa are adequate to trigger seismicity; (2) except at locations on the periphery of the reservoir, and at shallow depths within it (≤1 km), impoundment of the reservoir led to strengthening at hypocentral locations due to the undrained elastic effect; (3) diffusion of pore pressure is the dominant mechanism for the observed seismicity; and (4) the inferred permeability of the fractures associated with seismicity, 5 × 10–14 m2 (50 mD), lies within the range of seismogenic permeability associated with induced seismicity.