The species of the genus, commonly known as milk-caps, are characterized by the milky fluid ("latex") they exude when cut or damaged.

Lactarius Multifurca Russula Lactifluus Molecular phylogenetics uncovered that, while macromorphologically well-defined, milk-caps were in fact a paraphyletic genus; as a consequence, the genera Lactifluus was split from Lactarius, and the species L. furcatus was moved to the new genus Multifurca, together with some former Russula species.

[4][5][6] Phylogenetic analyses have also revealed that Lactarius, in the strict sense, contains some species with closed (angiocarpous) fruitbodies, e.g. L. angiocarpus described from Zambia.

The milk or latex emerging from bruised flesh is often white or cream, but more vividly coloured in some species; it can change upon exposition or remain unchanged.

[8] Microscopically, Lactarius species have elliptical, rarely globoid spores with amyloid ornamentation in the form of more or less prominent warts or spines, connected by ridges, like other members of the family Russulaceae.

Characters important for identification of milk-caps (Lactarius and Lactifluus) are:[13][14][15] initial colour of the latex and color change, texture of cap surface, taste (mild, peppery, or bitter) of latex and flesh, odor, and microscopical features of the spores and the cap cuticle (pileipellis).

[20] Several species have also been introduced with their host trees outside their native range,[21] e.g. in South America,[22] Southern Africa,[16] Australia,[23] and New Zealand.

A well-studied example is that of alders, which have several specialized Lactarius symbionts (e.g. L. alpinus, L. brunneohepaticus, L. lilacinus), some of which even evolved specificity to one of the Alnus subgenera.

[29] Other examples of specialized associations of Lactarius are with Cistus shrubs (L. cistophilus and L. tesquorum),[27][28] beech (e.g. L. blennius), birches (e.g. L. pubescens), hazel (e.g. L. pyrogalus), oak (e.g. L. quietus), pines (e.g. L. deliciosus), or fir (e.g. L. deterrimus).

[16] Lactarius species are considered late-stage colonizers, that means, they are generally not present in early-colonizing vegetation, but establish in later phases of succession.

L. deliciosus notably ranks among the most highly valued mushrooms in the Northern hemisphere, while opinions vary on the taste of other species, such as L. indigo or L. deterrimus.

Bitter or peppery species, for example L. torminosus, are generally not considered edible, at least raw, but are nevertheless consumed in some regions, e.g. in Finland.