Symbiotic relationships between plants and fungi are extremely common in nature, ranging from highly parasitic to closely mutualistic. Grasses, which are common and ecologically important components of many ecosystems worldwide, are often infected by clandestine, endosymbiotic fungi that grow within their stems, leaves, and seeds. This book attempts to synthesize the accumulating literature on grass-endophyte symbioses within a modern ecological and evolutionary framework. Topics covered include effects of endophytes on host growth, physiology, reproduction, and competitive ability in both agronomically important forages such as tall fescue and perennial ryegrass and in native grasses. Also, endophyte-host interactions are explored in relation to abiotic (e.g., drought) and biotic stresses (e.g., herbivory). Possible effects of endophyte infection on community and ecosystem-level processes are discussed. The ecological outcomes and coevolutionary dynamics of grass-endophyte associations are shown to be highly contingent on host and endophyte genotypes as well as environmental conditions. In addition to synthesizing much of the current literature on grass-endophyte interactions in natural and managed habitats, this book highlights gaps in current knowledge of specific aspects of symbiosis ecology and suggests many avenues for future research. Endophytic fungi are common in plants yet the nature of these interactions and how they cascade upward to communities and ecosystems are largely unknown.