Lichens are the result of symbiosis between a fungus called mycobiont and a photosynthetic partner called photobiont, which is either a green alga or a cyanobacterium, or both. This association offers extraordinary organisms that synthesize specific lichenic metabolites and that are endowed with biological activities and several applications in various fields. This book first focused on the identification of lichenic species collected from northern Tunisia. This taxonomic identification of lichens was done through macroscopic observations and chemical tests. Then, a quantitative study of the organic extracts and the extracts prepared by the quencher approach, was carried out by determining the content of phenolic compounds. The results showed that the extracts prepared by the quencher approach contain the highest contents of total polyphenols. In fact, all the tested extracts have important antioxidant and antimicrobial activities. Moreover, the licheno-chemical analysis reveals a great diversity of chemical composition.