Storage ring-based light sources will continue to play a vital role in X-ray science into the future since they offer beam properties that are complementary to FEL sources. Ring-based sources provide highly stable photon beams having low peak brightness with high average brightness and pulse repetition rate, photons that do not over-excite or damage samples the way those from FELs do, and they serve a large number of diverse users simultaneously. There are now emerging scientific applications and experimental methods that would greatly benefit from ring-based sources having much higher brightness and transverse coherence than present or near-future storage ring facilities – storage rings having electron emittance < ~100 pm-rad in both transverse planes – on the scale of the diffraction limited emittance for hard X-rays. Several institutions world-wide are now including the prospect of building diffraction-limited “ultimate” storage rings (USRs) in their 10-year development plans. These machines push the state of the art for storage ring accelerator and photon beam line design, presenting many significant challenges that must be addressed with R&D.