Mycena is a large genus of small saprotrophic mushrooms which are rarely more than a few centimeters in width. They are characterized by a white spore print, a small conical or bell-shaped cap, and a thin fragile stem. Most are grey or brown, but a few species have brighter colors. Most have a translucent and striate cap, which rarely has an incurved margin. The gills are attached and usually have cystidia. A few of the species exude a latex when the stem is broken, and many have the odor of bleach.
Mycenas are hard to identify to species and some are distinguishable only by microscopic features such as the shape of the cystidia. Some species are known to be edible, while some are known to contain toxins, but most of them are not known, as they are too small to be useful in cooking. Mycena cyanorrhiza stains blue and contains the hallucinogen psilocybin and Mycena pura contains the mycotoxin muscarine. Thirty-three species are known to be bioluminescent, creating a glow known as foxfire. These species are divided among 16 lineages, leading to evolutionary uncertainty in whether the luminescence developed once and was lost among many species, or evolved in parallel by several species. The evolutionary purpose of the glowing is uncertain.
There are 232 species identified in Alexander Smith's 1947 monograph about the genre, and the genus is now known to include about 500 species worldwide. Maas Geesteranus divided the genus into 38 sections in 1992, providing keys to each for all the species of the Northern Hemisphere. Many new species have been discovered since then, and four new sections have been proposed. Taxonomy is complex, as most sections are not truly homogeneous, and the keys fail for some species, especially those which may satisfy some criteria for only some of their life cycle. Some sections contain only one species.