Dike intrusion is a fundamental process of crustal accretion at mid-ocean ridge spreading centers. Although many studies of ophiolites and spreading centers treat dikes as passive infillings of tensile fractures, observations and mechanical models from subaerial rift zones demonstrate that widespread, temporally and spatially complefc mechanical effects accompany dike intru-sion. These effects can potentially have a profound influence on the proc-esses attending seafloor spreading. Specifically, dike intrusion should change the magnitude and orientation of the local stress field as a dike tip propagates along a spreading center. Consequently, dike emplacement could cause sig-nificant short-term (days to months), long-term (years to decades), and per-manent changes in the structure of the axial crust. The mechanical effects o f intrusion of even a single dike from a subaxial magma chamber should in-fluence the entire narrow (<5 km) axial rise and axial depression at fast-spreading ridges. A dike of the same size at a typical slow-spreading ridge should affect only a portion of the wide (>10 km) median valley. At fast-spreading ridges the effects of intrusion are associated with recent mi-croearthquake swarms, graben subsidence, fissures, eruptions, and hy-drothermal megaplume events. In contrast, relatively infrequent dike intrusions with only local effects on ridge-axis morphology and geology may be reflected by scattered volcanic edifices and large-scale faulting at slow-spreading ridges. We suggest that repeated dike intrusion will create systematically contrasting crustal assemblages that should reflect the varying relationship between faulting and magmatism at different mid-ocean ridge spreading centers.