CASTOR (the Cosmological Advanced Survey Telescope for Optical and uv Research) is a proposed CSA-led mission that would carry out deep, high-resolution imaging at ultraviolet and blue-optical wavelengths. Operating close to the diffraction limit, the 1m CASTOR telescope would have a spatial resolution comparable to the Hubble Space Telescope (HST), but with an instantaneous field of view of 1.2° x 0.6° -- about two hundred times larger than that of the Advanced Camera for Surveys on HST. Imaging would be carried out simultaneously in three non-overlapping bandpasses: UV (0.15-0.3 μm), u′ (0.3-0.4 μm) and g (0.4-0.55 μm). In the blue-optical region, CASTOR imaging would far exceed that from LSST in terms of depth and angular resolution, even after a decade of LSST operations. In this review, we summarize the various technical efforts being carried out in support of the CASTOR mission concept, and describe the potential scientific synergy between the CASTOR, Euclid and WFIRST missions.