Time-lapse 3-D seismic monitoring of subsurface rock property changes incurred during reservoir fluid-flow processes is an emerging new diagnostic technology for optimizing hydrocarbon production. I discuss the physical theory relevant for three-phase fluid flow in a producing oil reservoir, and rock physics transformations of fluid-flow pressure, temperature and pore-fluid saturation values to seismic P-wave and S-wave velocity. I link fluid-flow physical parameters to seismic reflection data amplitudes and traveltimes through elastic wave equation modeling and imaging theory. I demonstrate in a simulated data example that changes in fluid-flow can be monitored and imaged from repeated seismic surveys acquired at varying production calendar times.