Lichens are complex life forms resulting from a symbiotic partnership between multiple organisms, usually a fungus and an alga or cyanobacteria. Though often overlooked or mistaken for other organisms like moss, lichens are a critical part of each ecosystem they inhabit and are important sources of food, absorbers of carbon dioxide and biomonitors that help scientists detect air pollutants, among their many other functions. The Secret World of Lichens is a beginner naturalist's guide to the wonderful world of lichens.