Abstract: Ӣ Microbes feature as some of the most abundant and important mutualists with plants. In grasses, systemic fungal endophytes can ameliorate both abiotic stress (e.g., drought) and biotic stress (e.g., herbivory), and thereby promote host performance. I have used this system to address several ecological and evolutionary questions about the dynamics and consequences of symbiosis. First, to what extent do mutualistic symbioses affect community structure? Field experiments have revealed strong, cascading effects of these mutualisms on plants, arthropods, and other microbes. Effects are not easily predicted from the responses of individual community members, and detection requires a community- level perspective. While it remains unclear how widespread the community and ecosystem consequences of grass-endophyte symbioses may be, recent studies spanning several native grass and fungal species suggest these symbioses may commonly produce strong ecological impacts. Second, how can endophyte symbioses inform our understanding of the evolution of mutualisms? Is phylogenetic evidence consistent with co-speciation? Is there evidence for mutualism breakdown? How have evolutionary conflicts been resolved, and what factors may help stabilize mutualism?