Abstract: High-frequency assimilation and forecast (Rapid Update Cycle, RUC or Rapid Refresh, RR) have been developed for numerical weather prediction of short-range forecasting in recent years. In this paper, the history and status of RUC (or RR) are reviewed. Several relevant key issues are addressed, including: 1) adiabatic digital fi lter initialization (DFI) and diabatic digital fi lter initialization (DDFI); 2) cold start、warm start and hot start initialization techniques; 3) cloud analysis technique; 4) issues for assimilating nearsurface observations. Finally, the challenges, opportunities and future directions for rapid refresh systems are discussed.