Mycorrhizae are mutualisms between plants and fungi that evolved over 400 million years ago. This symbiotic relationship commenced with land invasion, and as new groups evolved, new organisms developed with varying adaptations to changing conditions. Based on the author's 50 years of knowledge and research, this book characterizes mycorrhizae through the most rapid global environmental changes in human history. It applies that knowledge in many different scenarios, from restoring strip mines in Wyoming and shifting agriculture in the Yucatán, to integrating mutualisms into science policy in California and Washington, D.C. Toggling between ecological theory and natural history of a widespread and long-lived symbiotic relationship, this interdisciplinary volume scales from structure-function and biochemistry to ecosystem dynamics and global change. This remarkable study is of interest to a wide range of students, researchers, and land-use managers.