SAFOD is a 4-km-deep observatory drilled directly into the San Andreas fault zone near the nucleation point of the 1966 magnitude 6 Parkfield earthquake. The project will reveal the physical and chemical processes acting deep within a seismically active fault. Initially, fault-zone rocks and fluids will be retrieved for laboratory analyses, and intensive downhole geophysical measurements will be taken within and adjacent to the active fault zone. The observatory’s long-term monitoring activities will include decades of detailed seismological observations of small- to moderate-sized earthquakes, and continuous measurement of pore pressure, temperature and strain during the earthquake cycle. SAFOD will provide direct information on the composition and mechanical properties of faulted rocks, the nature of stresses responsible for earthquakes, the role of fluids in controlling faulting and earthquake recurrence and the physics of earthquake initiation and growth. Drilling, sampling, downhole measurements and long-term moni- toring will allow testing of a wide range of hypotheses about faulting and earthquake generation, and the pursuit of a scientific basis for earthquake hazards assessment and prediction.