Cyathus is a genus of fungi in the Nidulariaceae, which is a family collectively known as the bird's nest fungi. They are given this name as they resemble tiny bird's nests filled with "eggs" – structures large enough to have been mistaken in the past for seeds. However, these are now known to be reproductive structures containing spores. The "eggs", or peridioles, are firmly attached to the inner surface of this fruit body by an elastic cord of mycelia known as a funiculus. The 45 species are widely distributed throughout the world and some are found in most countries, although a few exist in only one or two locales. Cyathus stercoreus is considered endangered in a number of European countries. Some species of Cyathus are also known as splash cups, which refers to the fact that falling raindrops can knock the peridioles out of the open-cup fruit body. The internal and external surfaces of this cup may be ridged longitudinally ; this is one example of a taxonomic characteristic that has traditionally served to distinguish between species.

The Nidulariaceae are a family of fungi in the order Agaricales. Commonly known as the bird's nest fungi, their fruiting bodies resemble tiny egg-filled birds' nests. As they are saprobic, feeding on decomposing organic matter, they are often seen growing on decaying wood and in soils enriched with wood chips or bark mulch; they have a widespread distribution in most ecological regions. The five genera within the family, namely, Crucibulum, Cyathus, Mycocalia, Nidula, and Nidularia, are distinguished from each other by differences in morphology and peridiole structure; more recently, phylogenetic analysis and comparison of DNA sequences is guiding new decisions in the taxonomic organization of this family.