For graduate and upper-level undergraduate students, an introduction to the differences and commonalities among a diverse range of symbioses involving animals, plants, fungi and prokaryotes and the ways in which they influence biological communities. Topics include the human gut microbiome, mycorrhizas, plant-rhizobium symbiosis, plant-insect symbioses, marine invertebrate-bacterial symbioses, ant symbioses, lichens and soil crusts.