The successful use of deep-depletion silicon CCDs as single photon x-ray detectors is described. CCDs used as x-ray detectors offer the unique combination of high spatial resolution and good non-dispersive spectral resolution. Deep-depletion CCDs additionally offer significant improvements in x-ray quantum efficiency. At an x-ray energy of 5.9 keV, we have measured a quantum efficiency of 76% for a 56 PM deep, 4000 ohm-cm RCA CCD, more than three times that of a conventional CCD (10 pm, 10 ohm-cm). Our tests with this RCA deep-depletion CCD have demonstrated good charge transfer efficiency (0.9999), at an operating temperature of -80 C. Dark current generation is -0.4 e/pixel/s at this temperature, sufficiently low for x-ray astronomy applications. Results of optical tests to measure system gain and CCD readout noise are also presented and discussed. Because the amount of charge per event generated by minimum ionizing cosmic rays in a deep-depletion CCD is > 5 times that of a conventional CCD, energy discrimination can be used to reject -90% of the charged particle background in a deep-depletion CCD. The high x-ray quantum efficiency, high spatial resolution, and ability to reject charged particle background make the deep-depletion CCD one of the strongest contenders for focal plane instrumentation on AXAF, the Advanced X-ray Astrophysics Facility.