Papillomatous digital dermatitis (PDD) is an infectious, superficial dermatitis that primarily affects the feet of dairy cattle. PDD is associated with intensive housing of cattle and prolonged exposure of bovine feet to moist, unhygienic conditions. Experimental transmission of PDD has been achieved by inoculating the feet of cattle with lesion material. Treatment of the lesions with antibiotics results in rapid resolution, consistent with the bacterial etiology of the disease. PDD lesions contain a mixed population of predominantly anaerobic bacteria. A pathognomonic feature of PDD lesions is the presence of large numbers of spirochetes invading deep into the epidermis, whereas other bacteria are located more superficially. It is currently unclear whether the PDD-associated spirochetes are primary pathogens or secondary opportunists. A number of Treponema phylogenetic groups have been demonstrated in PDD lesions. Of the spirochetes that are cultivable, only Treponema brennaborense has been classified as a new species. The remaining cultivable Treponema are closely related to human-associated T. denticola, T. medium/vincentii, and T. phagedenis. Cattle affected by PDD develop an immune response to Treponema antigens. A bacterin incorporating cultivable PDD-associated Treponema has been shown to be effective at reducing lesion development. PDD-associated Treponema have been recently demonstrated in cases of bovine ulcerative mammary dermatitis and contagious ovine digital dermatitis. Little is known concerning the origin and pathogenic mechanisms of the PDD-associated Treponema. Further research is required to clarify the taxonomic position and basic biology of these spirochetes.