Xystodesmidae is a family of millipedes. Its members often have very small distributional areas, with many species only known from a single locality. They are found across the northern hemisphere, with peak diversity in the Appalachian Mountains, where one-third of the 300 or so species occur. They are particularly abundant in deciduous broadleaf forests in the Mediterranean Basin, Africa, Asia, Central and North America, and Russia. Information on basic taxonomy is scant for this family; for example, it is estimated that the genus Nannaria contains over 200 species, but only 25 have so far been described.
Xystodesmidae contains many colorful and distinctive species, including Apheloria virginiensis of the eastern U.S. and Harpaphe haydeniana of the western U.S. The Sierra luminous millipedes of the genus Motyxia exhibit the only known examples of bioluminescence in the Polydesmida. Species of Apheloria and Brachoria in the Appalachians exhibit Müllerian mimicry, in which unrelated species resemble one another where they co-occur.
The family Xystodesmidae was named by O. F. Cook in 1895, upon the description of Xystodesmus martensii, (previously Polydesmus martensii). Xytodesmidae is placed in the suborder Chelodesmidea within the order Polydesmida (the "flat-backed" or "keeled millipedes"). Xystodemids are characterized by a relatively broad and compact body shape and one or more spines on the second leg-segments (prefemoral spines) in most species.