Lichens are the object of investigation within the framework of the BIOTA Southern Africa project, subproject S04 (http://www.biota-africa.org). This interdisciplinary research project, installed in 2000, focuses on the analysis of biodiversity and its changes along climatic and vegetation gradients (transects) in Namibia and in the Republic of South Africa. In the context of this project, studies on the diversity of lichens are carriedout. Special reference is given to the monitoring of lichens growing on soil, which form the so called biological soil crusts.Lichen diversity is assessed and analysed with respect to its spatial and temporal changes. These are related to various abioticand biotic factors such as climate, soil features and land use. The indicator value of certain terricolouslichen taxaand/or lichen groups (communities) is investigated for the study area, and it is intended to use itin a future long-term monitoring programme in the region. In this brochure, we whish to explain what lichens are, how do they live and where do they grow, and why they are so important as bioindicatorsin arid and semi-arid areas of the world. The activities of the S04 subproject along the BIOTA transect are described, as well as the methods used for monitoring environmental changes in Southern Africa using soil-inhabiting lichens.