A practical field guide to the common lichens found in the northeastern megalopolis, including New York City, Toronto, Boston/New Haven, Philadelphia, Baltimore/Washington, D.C., and as far west as Chicago Lichens are dynamic, symbiotic organisms formed by close cooperation between fungi and algae. There are over 20,000 identified species performing essential ecosystem services worldwide. Extremely sensitive to air pollution, they have returned to cities from which they were absent for decades until the air became cleaner. This guide is the first to introduce urban naturalists to over 60 of the common lichens now found in cities and urban areas throughout northeastern North America--in parks and schoolyards, on streets, and in open spaces. Divided into three sections -- lichen basics, including their biology, chemistry, morphology, and role in human history; species accounts and descriptions; and an illustrated glossary, index, and references for further reading -- the book aims to connect city dwellers and visitors with the natural world around them. The descriptions, exquisite photographs, and line drawings will enable users to enter the hidden world of lichens.