The Graduation Thesis presents the method of inventory of epiphytic lichens for the assessment of the environmental condition in the forest area unit of Postojna. The first part of the Thesis is the presentation of the regional unit of Postojna. Then it is followed by the explanation of the method of the inventory of epiphytic lichens - assessing the presence and cover of lichens on tree species on particular surfaces. The results are given by calculations of median relative cover, which helps for the intepretation of the state of air pollution and the state of the environment. The results show the state of cover as worse environmental condition, but there is a question if the actual state is really so bad or only satisfactory. The reasons for the poor results lie in the method and implementation of it. The general statement was that the environmental situation in the forests in the area of the research, can not be fully determined by this method.