Xerophilic fungi are yeasts and moulds that are capable of growth at or below a water activity (aw) of 0.85. These microorganisms have developed physiological mechanisms that enable their biochemical pathways to function in environments where little water is available. External conditions of low aw are sensed by membrane osmosensors, and xerophiles then accumulate glycerol as a compatible solute to balance the internal and external osmotic pressure. They also modify their membranes to retain this glycerol within the cell. As a group, xerophiles are extremely important in the spoilage of many processed foods and stored commodities, and in indoor environments. Moderate xerophiles include species within Aspergillus, Penicillium and Eurotium. Extreme xerophiles compete poorly at high aw, because they require decreased aw for growth. Some xerophiles have a preference for salt or sugar substrates, whereas other species can be isolated from both jam and salterns. Xerophiles are widely spread on the fungal tree of life