  We review the current state of tabletop extreme ultraviolet (XUV) sources
based on high harmonic generation (HHG) in femtosecond enhancement cavities
(fsEC). Recent developments have enabled generation of high photon flux (1014
photons/sec) in the XUV, at high repetition rates (>50 MHz) and spanning the
spectral region from 40 nm - 120 nm. This level of performance has enabled
precision spectroscopy with XUV frequency combs and promises further
applications in XUV spectroscopic and photoemission studies. We discuss the
theory of operation and experimental details of the fsEC and XUV generation
based on HHG, including current technical challenges to increasing the photon
flux and maximum photon energy produced by this type of system. Current and
future applications for these sources are also discussed.
