Varying degrees of water pollution have been found in lots of reservoirs built between the 1950s and 1960s in China after operation for several decades. Compared with point-source pollution and non-point source pollution, internal pollution is more difficult to find and control. Based on review of worldwide research on internal pollution, the cause of rapid formation of internal pollution of reservoirs is discussed from three aspects: the source, the discharge routes, and the factors of accumulation of pollutants. The impacts of internal pollution on the water quality of reservoirs are analyzed. It is pointed out that adaptive samplers, monitoring of internal pollutant transport in and out of reservoirs, distribution of pollutants, the sink-source mechanism of internal pollution, and internal pollution treatment technology should be further studied.