Lichens exhibit a broad spectrum of biological phenomena that are either directly or indirectly influenced by light. The aim of this study is to analyze and explain patterns in the photobiology of lichens, with special emphasis on lichen pigments and their photobiological functions. Here the primary purpose is to rationalize the role of light in explaining ecological and coevolutionary phenomena in lichen symbioses. This objective is pursued by investigating regularities in the light responses of lichens and lichen bionts over a wide range of spatial, temporal and organizational scales. The study is mainly based on library sources. Due to the multidisciplinary nature of the subject matter, the results are presented in six chapters, grouped in two entities, both investigating the photobilogy of lichens from a slightly different perspective. The three first chapters concentrate on factors which can be described as forming the photobiological framework of lichens, while the latter three chapters give detailed examples of how these factors are expressed in the distribution and ecology of lichens. As far as possible the chapters are intended to also act independently. Numerous topics concerning various aspects of the role of light in the evolution, distribution, morphology, anatomy, chemical composition, ecology and physiology of lichens and lichen bionts are discussed, and special attention is directed to non-nutritive interactions related to light availability as a central selective advantage of being lichenized. The study underlines the great diversity of ways in which lichen bionts and lichen symbioses have adapted to different light regimes. Even in the case of seemingly uniform groups of lichens, such as the lichenized Caliciales of boreal forests, the influence of light as an ecological factor becomes evident on a range of spatial and temporal scales, these ranging from interbiont relations to the correlations between lichen distributions and forest dynamics or global climate. It seems that from the photobiological perspective the ecological and coevolutionary essence of lichen symbioses may never quite bend to taxonomically oriented definitions, and lichens should rather be seen as symbiotic processes. It also appears that the photobiological coadaptation of lichens may form a fruitful base for studying, not only individual lichens, but also lichen assemblages as symbiotic processes. This approach could help to elucidate ecological parameters that have remained hidden in previous studies greatly emphasizing the role of vegetational analysis in the study of lichen vegetation. Key words: Lichen, symbiosis, mutualism, symbiotic processes, coevolution, terrestrialization, non-nutritive interactions, lichen communities, Caliciales, Parmeliopsis, Xanthoria, Chlorophyta, Trebouxia, Trentepohlia, cyanobacteria, Nostoc, photobiont, photobiology, photosynthesis, light-harvesting, UV-B, ozone depletion, photoinhibition, photodamage, photoprotection, pigments, carotenoids, phycobilins, photoreceptors, lichen compounds, melanins, winter ecology, cold resistance, psychrophily, chionophily, cheimophotophytic lichens, open shade, pored epicortex, oilbodies, oil-hyphae.