RF Superconductivity -- An Essential Technology for 4$^{th}$ Generation Light Sources

The 4$^{th}$ generation light sources, which are expected to produce high brilliance photon beams, will require very low emittance electron beams. This will put stringent requirements and demands on the electron sources and on the accelerators needed to produce them. The characteristics of the superconducting rf (SRF) technology makes it ideally suited for such facilities, whether free electron lasers or energy recovering linacs. The opportunities offered by the SRF technology for 4$^{th}$ generation light sources, and its challenges, will be presented and reviewed.