The lichens diversity of the Republic of Moldova has been studied sporadically over the past 75 years. Most of the publications concerned epiphyte species inhabiting natural reserves and forest ecosystems located in the central part of the country. These type of ecosystems cover a rather small part (c.11%) of the territory and are subject to heavy local and trans-boundary anthropogenic impact. Most lichens are xtremely sensitive to increased concentrations of SO2, NOx and other gases: the damage is reflected in reduced photosynthesis, necrosis and even thallus destruction. Currently, the lichen diversity of the Republic of Moldova comprises 197 species and varieties, of which 23 species are reported nationally for the first time and 40 species are recommended as reliable indicators. The paper presents a Lichens Toxitolerance Scale (LT S) towards SO2 concentration in the air and Air Quality Assessment Scale (AQAS) with 6 gradations, taking into account the specific diversity, abundance and indicator of species toxi-tolerance.