From old growth forests to the human body, fungi are so primary to bionetworks that they interact in some way with every species on Earth. These interactions take on a myriad of forms, with fungi playing roles as deadly pathogens, obligate mutualists, essential carbon cyclers, and biological workhorses responsible for the production of goods as diverse as antibiotics, industrial enzymes, beer and bread. Across this diversity, fungi span scales of community organization and tie molecules to ecosystems by forming networks of interspecific interactions. Understanding how these interactions are formed, maintained, and broken, is essential to making epidemiological predictions for both plant and animal hosts, to preserving biodiversity, and to forecasting how ecosystems will respond to a changing planet.