Since the appearance of the first volume of Advances in Microbial Ecology in 1977 under the editorship of Martin Alexander, the series has achieved wide recognition as a source of in-depth, critical, and sometimes provocative reviews on the ecology of microorganisms in natural and man-made ecosystems. Most reviews published in Advances have been prepared by experts at the invitation of the Editorial Board. The Board intends to continue its policy of soliciting reviews, but individuals are encouraged to submit outlines of unsolicited con tributions for consideration of their suitability for publication in Advances. Volume 7 of Advances in Microbial Ecology covers a range of topics related to the ecology of microorganisms in natural and artificial habitats. R. M. Atlas discusses the measurement and significance of diversity in microbial communities. The nature of deserts and the activity of microorganisms in desert soils are considered by J. Skujil)s. D. B. Nedwell examines both the input and the mineralization of organic carbon in anaerobic aquatic sediments. The role of microcosms in the evaluation of interactions between pollutants and microorganisms is the basis of a major review by P. H. Pritchard and A. W. Bourquin."