Faults are an efficient mechanism for allowing large strains to accumulate in the upper lithosphere of the Earth and other planetary bodies. In general, faults are arrays of 3-D surfaces along which large shear offsets (fault-parallel displacements) have been accommodated by frictional sliding (see Chapter 1). Faults can redirect the flow of subsurface fluids (either channeling or restricting the flow), modify the transmission of seismic waves, and create spectacular surface topography—through steady or destructive slip events—that have attracted mankind over millennia for the associated resources and natural beauty.