[2] The genus was circumscribed in 1930 by German lichenologist Friedrich Tobler, with Herpothallon sanguineum assigned as the type species.

[5] In 2009, Herpothallon was resurrected following a publication by André Aptroot, Göran Thor, Robert Lücking, and John Elix, in which they recognized 29 species worldwide.

[8] Herpothallon is characterized by the byssoid (a wispy or cottony texture) prothallus (i.e., the first purely fungal layer upon which an algae-containing thallus develops) and hypothallus (i.e., a growth of undifferentiated purely fungal mycelium present as a distinct layer on the underside of the thallus).

Another thallus feature common to all species is the felty pseudoisidia; these are isidia-like outgrowths that, unlike true isidia, lack internal differentiation and have no distinct cortex.

The thallus also has pustules (blister-like elevations), and granules ranging in form and size from soredia-like to minute.