Marcus, G. JACoW Publishing Geneva, Switzerland Cavity-based X-ray free-electron laser concepts such as the regenerative amplifier FELs (RAFEL) and the X-ray FEL oscillator (XFELO) offer a promising path towards enhancing the brightness of XFELs by up to two orders of magnitude. We will present the ongoing research effort at the SLAC National Accelerator Laboratory and Argonne National Laboratory to demonstrate these concepts. We will discuss recent experimental tests of X-ray cavity performance and present plans for a proof-of-principle experiment at LCLS, as well as ideas for future cavity-based XFEL facilities.