This document provides an overview of lichens, which are symbiotic associations between fungi and photosynthetic organisms such as algae or cyanobacteria. It discusses the structure and types of lichens, including their thallus morphology and internal structure. It also covers the fungal and photosynthetic partners involved, lichen reproduction, growth factors, the symbiotic relationship between partners, and the economic importance of lichens as a food source, forage, in medicine, dyes, perfumes, and as bioindicators of air pollution.