Abstract
High harmonic generation (HHG) by passive femtosecond enhancement cavities is reviewed with a detailed example studied at JILA. The technology is aimed towards generation of phase-coherent VUV radiation that maintains a one-second coherence time of fundamental pulses. The basic strategy to couple fs pulses into high-finesse optical cavities and the difficulties encountered in doing so are first summarized. Then, the current status, technical limitations, and future outlook of this novel HHG are described.