Studies at CERN on niobium-coated 1.5 GHz superconducting cavities are aimed at understanding and possibly curing the causes of the residual resistance increase observed when increasing the accelerating gradient above 15 MV/m. Amongst the possible causes, the surface defects and roughness, the grain size, the hydrogen content and the thermal effects are currently being investigated. The present status of understanding, together with some recent results on high field operation, are presented and discussed
