A key component in healthy ecosystems, lichens can be found in almost any natural habitat in the Pacific Northwest. This revised and expanded edition of the first comprehensive guide to the region's macrolichens is intended for use by beginners as well as specialists: weekend naturalists will be able to identify specimens and recognize the great diversity of lichens, while lichenologists and mycologists will gain greater knowledge of the distribution and abundance of various species. The new edition incorporates an understanding of macrolichens that has advanced tremendously in the ten years since the first edition. It includes 116 new species, many of them new to science since 1997, and 176 additional illustrations. Macrolichens of the Pacific Northwest includes keys to 108 genera and 574 species of Oregon and Washington macrolichens all the macrolichens known or expected to occur in the two states. The keys also provide reasonable coverage for lichens of Idaho and Montana, inland to the Continental Divide. Color photographs and detailed descriptions are provided for 243 species, emphasizing lichens prevalent in forested ecosystems. The illustrated glossary and introductory material cover the terminology needed to identify macrolichens and provide information on collection and handling. The biology, ecology, and air-quality sensitivity of lichens are discussed, and regional air quality sensitivities are provided for more than one hundred species. Macrolichens of the Pacific Northwest will prove invaluable to anyone seeking to identify lichens or to better understand these organisms and their vital role in the natural world.