Xerophiles are extremophilic organisms that can grow and reproduce in conditions with a low availability of water, also known as water activity. Water activity (aw) is a measure of the amount of water within a substrate that an organism can use to support growth. Xerophiles are often said to be "xerotolerant", meaning tolerant of dry conditions. They can survive in environments with water activity below 0.8. Endoliths and halophiles are often xerotolerant.
The common food preservation method of reducing water activities may not prevent the growth of xerophilic organisms, often resulting in food spoilage. Many mould and yeast species are xerophilic. Mould growth on bread is an example of food spoilage by xerophilic organisms.