Abstract: Recent progress in understanding the haze formation over the Beijing-Tianjin-Hebei area was systematically reviewed, including the long-term trend in haze days, the synoptic characteristics during haze formation periods, and source apportionment of the primary pollutant, PM2.5. The studies show that haze days in Beijing began to decline from 2000, as well as the mass concentration of PM2.5. However, the annual PM2.5 mass concentration for 2013 in Beijing was 89.5 μg m^-3, i.e., almost three times the national standard (35 μg m^-3). Thus, the heavy pollution over the Beijing-Tianjin-Hebei area demands effective control. Studies also show that the frequent occurrence of haze over the Beijing-Tianjin-Hebei area is caused by a combination of stable synoptic conditions and heavy pollutant emissions. The regional transport of pollutants plays an important role in regional haze episodes, and local atmospheric boundary layer processes over the Beijing-Tianjin-Hebei area (mountain-valley breezes, sea-land breezes, and urban heat island) can result in accumulation of pollutants and exacerbating the local air quality. Additionally, a two-way feedback process between atmospheric boundary layer evolution and pollutants accumulation contributes substantially to the haze formation. To control the heavy pollution over the Beijing-Tianjin-Hebei area, joint efforts among the cities in this cluster are necessary.