Lichens dominate ca. 8% of the world's terrestrial ecosystems and play important ecological roles in many other ecosystems, such as foliicolous lichens in the tropics that provide a model system for studying the development of their symbiotic relationship in the fi eld. In any discussion of symbiosis, the lichenized fungi play a prominent role. Not only is a lichen a combination of fungus and alga(e) and/or a cyanobacterium, but also hosts specifi c bacteria and potentially other lichenicolous fungi. Accordingly, this volume of Bibliotheca Lichenologica opens with a modern review of symbiosis and closes with a paper on the selectivity of the photobiont. The possible origin of the symbiosis from a paleoecological perspective is also considered. The ecological role of lichens in the widely distributed, arid soil crusts, as well as several specifi c aspects of their community ecology, are also discussed in other chapters. For nearly two centuries lichens have been used in environmental monitoring, especially in relation to air pollution, new aspects of such monitoring relative to cumene hydroperoxide, Hg, NH3, O3, and various organopollutants are covered in separate chapters. Several systematic papers dealing with the Parmeliaceae and lichenicolous fungi are also presented. Cyber applications are now providing a wealth of information from literature records back to the 1500s to vastly improved identifi cation tools, to access to millions of collection records, and to integrating biodiversity networks.