Long-term observations of aerosol optical properties are critical for quantifying aerosol radiative forcing and climate effects, as well as for studying air quality and environmental health. This paper provides a systematic review of the global AERONET network and introduces the development and research achievements of the AEROENT Beijing station, which has the longest observation time in China. Based on more than twenty years of observation data from the Beijing station, the multi-time scale variability of optical radiation products is analyzed. The importance and urgency of long-term observations are also discussed.