Scientific research using lichens as indicators of the air environment is carried out based on research results from around the world, as well as surveyed and collected data in Bac Ninh province (the northern of Vietnam). The results of studying the biological and ecological characteristics of lichens show that lichens have a fairly simple structure and connection between the constituent parts of lichens. The life of lichens is associated with many types of substrate and direct absorption of substances in the air environment. Therefore, lichens are easily affected by factors in the atmospheric environment and expressed through their biodiversity indicators. Research around the world has determined the species composition, physiological and biochemical characteristics of lichens related to atmospheric environmental factors. Since then, there have been studies using lichens as indicator organisms to assess air quality. In Vietnam, research results in Bac Ninh province show that the correlation between the AQI_SO2 index and lichen coverage in habitats has a fairly close linear relationship and a negative correlation through the correlation coefficient R = -0.881. The diversity indices (d, H', J') of lichens have a less close linear relationship with the AQI_SO2 index as shown by the low correlation coefficient. This result shows that lichen biodiversity indices may have a nonlinear relationship with the AQI_SO2 index. These data are considered the scientific basis of research on using lichens as biological indicators of the air environment, specifically indicators of SO2 content in the air in some provinces/cities in Northern of Vietnam.