Practically fluids are involved into all geodynamic processes. The literature data on studies of subsurface fluids are evidence that fluids are governing factors that affect present-day geodynamical processes and seismicity. Earthquakes are mainly concentrated in the region of fault zones. Breaks defining the inner structure of these zones of any type serve as conductors for fluids of various origin and composition. Increase in seismic activity observed in regions of large storage water reservoirs are explained in the literature by effect of water on seismicity. However, the nature of effect of fluids on the process preceding earthquake and on earthquake realization remains obscure. For most of rocks and minerals, water is an adsorption-active medium with variable physical-mechanical properties: the magnitude of stresses provoking both fracture and plastic deformation of solids decreases (Rehbinder’s effect). When deep-focus earthquakes occur in zones of collision and subduction (up to a depth of 700 km), molten magma, which is formed due to friction when slabs are moved, affects physical-mechanical rock properties more effectively. Rocks of the hydrated crust being subducted (deep recycling) are partially melted that is favored by water entering into the composition of rocks and by decreasing melting temperature of the rocks. Thus, effect of fluids on seismicity results in decrease in the stress level at which seismic process is going on.