Hydnellum is a genus of tooth fungi in the family Bankeraceae (order Thelephorales). Widely distributed in the Northern Hemisphere, the genus contains around 40 species. The fruitbodies of its members grow by slowly enveloping nearby bits of grass and vegetation. There is great variability in the form of Hydnellum fruitbodies, which are greatly influenced by environmental conditions such as rainfall and humidity, drying winds, and temperature. They are too tough and woody to eat comfortably. Several species have become the focus of increasing conservation concern following widespread declines in abundance.

Hydnellum species produce pigments that have been used to dye textiles. Several chemical compounds—some with unique biological activity—have been isolated and identified from the genus.

One of the better-known species is the unusual pinkish-white Hydnellum peckii, also known as "strawberries and cream" or as the "bleeding tooth fungus" due to the red droplets that appear on the pinkish or whitish fruitbodies. Another species, H. suaveolens, has a strong odor of anise or peppermint.

Hydnellum was circumscribed by Finnish mycologist Petter Adolf Karsten in 1879 with what was then known as Hydnum suaveolens as the type species. Before then, fungi with spines (hydnoid fungi) had been grouped in Hydnum by Elias Fries in his 1821 work Systema mycologicum. Karsten defined Hydnellum as having fruitbodies with a corky or leathery, tough cap, and a centrally attached stipe. Synonyms of Hydnellum include Calodon (Karsten, 1881), and Phaeodon (Joseph Schröter, 1888).

Hydnellum is classified in the family Bankeraceae, which was circumscribed by Marinus Anton Donk in 1961. The genus was not in Donk's original family concept, which included only Bankera and Phellodon, genera whose species produce hyaline (translucent), and ornamented spores. Donk left Hydnellum in the tribe Hydnelleae of the family Thelephoraceae, along with Sarcodon and Hydnodon. In 1981, however, Walter Jülich emended Donk's concept of the Bankeraceae, adding hydnoid genera that produced brown, lobed spores—Hydnellum and Sarcodon.

The name originates from the Greek word ύδνο (=hydnum) meaning "spongy plant" or "fungus". The British Mycological Society, in their recommended list of common names for fungi in the United Kingdom, name Hydnellum fungi in the form "descriptor word" plus "tooth", such as "gold tooth" (H. auratile), "zoned tooth" (H. concrescens), and "velvet tooth" (H. spongiosipes).

Phylogenetic studies have shown that, in its current circumscription, the genus Hydnellum is polyphyletic and only distantly related to the white-spored genera Bankera and Phellodon, currently grouped together in the family Bankeraceae. Analyses of the ITS region by Parfitt and colleagues and Ainsworth and colleagues, have further revealed multiple cryptic lineages within the genus. Most of the cryptic species belong to the H. concrescens/H. scrobiculatum species complex, resulting in frequent misinterpretations of these old and widely applied names. At the moment, the number of phylogenetically distinct lineages confirmed by molecular sequencing exceeds the number of described taxa in the genus, making further taxonomic revisions inevitable in the future.

Hydnellum fruitbodies have caps and stipes, often with indeterminant growth forms, that may grow in spurts and decay over several weeks. Neighboring fruitbodies can coalesce, forming intricately intertwined caps and partially fused stipes. The flesh has a zoned appearance and is fibrous when fresh, but becomes hard and woody when dry. Zones in the flesh reflect differences in growth during periods of low daytime and high nighttime humidity, and give a fairly accurate record of daily growth. The spines are crowded closely together and are typically decurrent (extending down the length of the stipe). Fruitbodies may display a variety of colors, from white to yellow, olive green, shades of orange, light brown, or dark brown in age.

Spores of Hydnellum are almost spherical to oblong and tuberculate, and are brown in mass. The basidia (spore-bearing cells) are narrowly club-shaped and usually four-spored; there are no cystidia in Hydnellum. Three types of hyphae are found in the flesh of Hydnellum: generative hyphae (thin-walled, not inflated); skeletal hyphae (thick-walled and narrow); and thin-walled gloeoplerous-like hyphae, which stain with methyl blue.

In conditions of high humidity, several species can form striking colored drops on the actively growing caps: red drops in H. peckii, H. diabolus, H. ferrugineum, and H. cruentum, yellow drops in H. caeruleum, and coffee-colored drops in H. mirabile. The common names of H. peckii reflect its appearance: "strawberries and cream" and "bleeding tooth fungus". Some Hydnellum species have a mealy odor (e.g. H. mirabile and H. pineticola) similar to freshly ground flour. H. zonatum smells like melilot, while H. suaveolens has a sweet odor resembling anise or peppermint. All are too tough and woody to be edible, and many have an acrid taste anyway.

Differences between Hydnellum species tend to be more distinguishable in younger specimens. Fruitbody development is greatly influenced by environmental factors such as levels of rainfall, drying winds, and temperature. The blue tooth (H. caeruleum), for example, develops a deeper blue color when it grows during cooler autumn weather. Optimal growth occurs during periods of frequent light rains and high humidity; if the habitat dries out, growth will stop, but may resume after further precipitation. This intermittent growth affects the fruitbodies of different species to variable extents, leading to large variations in form, surface texture, and color. The morphological variability of fruitbodies and the dependence of their appearance upon environmental conditions has made Hydnellum a difficult group to study. Canadian mycologist Kenneth A. Harrison, who described several new species from North America, noted "[t]he remarkable longevity of individual sporophores of many species and the changes in appearance that occur during the long period of their development have confused all workers studying this group." For example, H. aurantiacum, initially white, becomes in turn shades of orange, rusty-brown, and brownish-black. Its fruitbody initially has a turbinate (cushion-like) shape with a lumpy surface, later becoming flattened to funnel-shaped with a smooth to corrugated surface texture.