Polydesmida (from the Greek poly "many" and desmos "bond") is the largest order of millipedes, containing approximately 3,500 species, including all the millipedes reported to produce hydrogen cyanide (HCN).

Members of the order Polydesmida are also known as "flat-backed millipedes", because on most species, each body segment has wide lateral keels known as paranota. These keels are produced by the posterior half (metazonite) of each body ring behind the collum. Polydesmids have no eyes, and vary in length from 3 to 130 mm. Many of the larger species show bright coloration patterns which warn predators of their toxic secretions. Adults usually have 20 segments, counting the collum as the first ring and the telson as the last ring. Juveniles have from 7 to 19 rings. In species with the usual 20 segments, adult females have 31 pairs of legs, but in adult males, the eighth leg pair (the first leg pair of the 7th ring) is modified into a single pair of gonopods, leaving only 30 pairs of walking legs. Many species deviate from the typical body plan. One striking and unique deviation occurs in adult males of the species Aenigmopus alatus, which retain 31 pairs of walking legs and feature no gonopods. This millipede is the only species in the infraclass Helminthomorpha without gonopods. The most common deviation, however, is a reduction in the number of segments. Many species have only 19 segments (including the telson) as adults, including those in the genera Brachydesmus, Macrosternodesmus, Poratia, and Bacillidesmus. In these species, adult females have only 29 pairs of legs, and adult males have only 28 pairs of walking legs. In a few species, including Hexadesmus lateridens, Agenodesmus reticulatus, and Eutynellus flavior, adults have only 18 segments (including the telson), with a corresponding reduction in the number of leg pairs (27 in the adult female, 26 in the adult male, excluding the gonopods). Still other species exhibit sexual dimorphism in segment number, for example, Prosopodesmus panporus (the usual 20 in adult females, but only 19 in adult males) and Doratodesmus pholeter (19 in adult females; 18 in adult males), with the expected number of leg pairs given the number of segments in each sex. A few species deviate by having more than the usual number of segments, including those in the genus Devillea. For example, in the species D. tuberculata, adult females have 22 segments and adult males have 21 (including the telson), with a corresponding increase in the number of leg pairs (35 in adult females and 32 in adult males, excluding the gonopods). Some species in this genus also exhibit variation in segment number within the same sex, for example, in D. subterranea, adult males can have as few as 19 segments or as many as 23 (including the telson). The most extreme outlier in segment number among polydesmids, however, is a cave-dwelling species discovered in Brazil, Dobrodesmus mirabilis, with adult males found to have 40 segments (including the telson).

Polydesmids are very common in leaf litter, where they burrow by levering with the anterior end of the body.

The c. 3500 species of Polydesmida are variously classified into four suborders (names ending in "-idea"), and 29 families, the largest (numerically) including Paradoxosomatidae, Xystodesmidae, and Chelodesmidae. - Dalodesmidea Hoffman, 1980 . 2 families - Dalodesmidae Cook, 1896 - Vaalogonopodidae Verhoeff, 1940 - Leptodesmidea Brölemann, 1916 . 13 families: - Chelodesmoidea Cook, 1895 - Chelodesmidae Cook, 1895 - Platyrhacoidea Pocock, 1895 - Aphelidesmidae Brölemann, 1916 - Platyrhacidae Pocock, 1895 - Rhachodesmoidea Carl, 1903 - Rhachodesmidae Carl, 1903 - Tridontomidae Loomis & Hoffman, 1962 - Sphaeriodesmoidea Humbert & de Saussure, 1869 - Campodesmidae Cook, 1896 - Holistophallidae Silvestri, 1909 - Sphaeriodesmidae Humbert & de Saussure, 1869 - Xystodesmoidea Cook, 1895 - Eurymerodesmidae Causey, 1951 - Euryuridae Pocock, 1909 - Gomphodesmidae Cook, 1896 - Oxydesmidae Cook, 1895 - Xystodesmidae Cook, 1895 - Paradoxosomatidea Daday, 1889 . 1 family - Paradoxosomatidae Daday, 1889 - Polydesmidea Pocock, 1887 . 12 families - Oniscodesmoidea Simonsen, 1990 - Dorsoporidae Loomis, 1958 - Oniscodesmidae DeSaussure, 1860 - Pyrgodesmoidea Silvestri, 1896 - Ammodesmidae Cook, 1896 - Cyrtodesmidae Cook, 1896 - Pyrgodesmidae Silvestri, 1896 - Haplodesmoidea Cook, 1895 - Haplodesmidae Cook, 1895 - Opisotretoidea Hoffman, 1980 - Opisotretidae Hoffman, 1980 - Polydesmoidea Leach, 1815 - Cryptodesmidae Karsch, 1880 - Polydesmidae Leach, 1815 - Trichopolydesmoidea Verhoeff, 1910 - Fuhrmannodesmidae Brölemann, 1916 - Macrosternodesmidae Brölemann, 1916 - Nearctodesmidae Chamberlin & Hoffman, 1958 - Trichopolydesmidae Verhoeff, 1910