The Agaricales are an order of fungi in the division Basidiomycota. As originally conceived, the order contained all the agarics, but subsequent research has shown that not all agarics are closely related and some belong in other orders, such as the Russulales and Boletales. Conversely, DNA research has also shown that many non-agarics, including some of the clavarioid fungi and gasteroid fungi belong within the Agaricales. The order has 46 extant families, more than 400 genera, and over 25,000 described species, along with six extinct genera known only from the fossil record. Species in the Agaricales range from the familiar Agaricus bisporus and the deadly Amanita virosa to the coral-like Clavaria zollingeri and bracket-like Fistulina hepatica.

A fungus is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as one of the traditional eukaryotic kingdoms, along with Animalia, Plantae and either Protista or Protozoa and Chromista.