Fungal endophytes are microfungi that inhabit living plants, but which are cryptic and asymptomatic in their hosts. Endophytes have been found in photosynthetic tissue in all lineages of land plants examined to date, and appear to be extremely diverse: tropical leaves typically contain dozens of species, and endophytes as a whole represent all major lineages of non-lichenized Ascomycota. Despite their ubiquity and apparent diversity, however, little is known regarding their phylogenetic relationships, host affinity, geographic structure, or ecological roles. The Arnold lab is surveying endophytic fungi in tropical, temperate, boreal, and arctic sites to examine the diversity, ecology, and phylogenetic relationships of these little-known but hyperdiverse symbionts, including both culturable and unculturable species. Concurrently, collaborators in Panama are focusing on secondary metabolites of these fungi, seeking pharmaceutical applications of tropical endophytes.