Cyathus is a genus of fungi in the family Nidulariaceae. Along with the genera Crucibulum, Mycocalia, Nidula and Nidularia, they are known collectively as the bird's nest fungi due to their small nest-like fruiting bodies containing egg-shaped peridioles. The genus Cyathus was monographed by mycologist Lloyd (1906), and later Brodie (1975, 1984), and their species concepts, especially those of Brodie (1975), are followed by most mycologists.