This paper describes the state-of-the-art precision bonding technique for cryogenic fused-silica optics. It was developed for assembling the fused-quartz Gravity Probe-B science instrument, which will be used to prove or disprove Einstein's Theory of General Relativity with unprecedented accuracy and precision. This room-temperature bonding process is based on hydroxide catalysis. The resulting bonding strength is comparable with that of fused silica or fused quartz. The interface is typically 200 nm essentially limited by surface figure mismatch. It is as precise as optical contacting, as reliable as high-temperature frit bonding, as transparent as optical epoxies. So far it is the only bonding approach that meets all the stringent requirements for GP-B's applications at 2.5 Kelvin.