The main objective of this chapter was to describe the physicochemical and biological characteristics of selected lichenized ascomycetes and the influence of their physiologically active compounds on human health, through scientifically proven information. The chapter presents the biologically active metabolites derived from lichen species (polyphenols, volatile compounds, lipids, phospholipids, fatty acids, and organic acids). Lichens and their metabolites have been demonstrated to possess numerous biological activities, including antiviral, antibacterial, antitumor, and enzyme inhibitory activity. The influence of environmental factors on the lipid and fatty acid composition of some lichen species has also been reported. Lichens are easily exposed to halogens, which are present in polluted air. We present chlorinated metabolites, which are isolated from various species of lichens. Chlorine is one of the main pollutants in nature. The structures of about 50 chlorinated metabolites of phenolic nature generated by lichenized ascomycetes are considered. Some active lichen substances are used in the pharmaceutical industry. Keywords: Lichens · Lichenized ascomycetes · Lipids · Fatty acids · Glycosides · Polar lipids · Activity · Chlorinated metabolites · Depsides · Depsidones.