"Extremophiles 2002," held in Naples, Italy, 22 to 26 September 2002, was the fourth international conference on microorganisms growing in very diverse and severe environments, such as high or low temperature, acidic and alkaline conditions, high salt concentrations, and high pressure. About 550 researchers from 30 countries discussed subjects including the ecology, phylogenetics and evolution, molecular and cell biology, genomics, proteomics, enzymology, and structural biochemistry of these fascinating microbes.