The family Syllidae Grube, 1850, comprises errant polychaetes of relatively small sizes, usually a few milimeters in length, but some of the largest representatives were reported as reaching lengths of up to 13 cm, with hundreds of segments (e.g. Trypanosyllis ingens).

Syllids may inhabit virtually all environments in the oceans, from the deep sea to the intertidal zones. Usually benthic organisms, most of them "switch" to a planktonic form, the epitoke, as part of reproductive cycles. For some species, epitokous forms are known to display strong bioluminescence, as is the case of the "Bermudan fireworm" Odontosyllis enopla.

Syllids posses a well-formed head, usually with 1 pair of triangular to kidney-shaped palps, 3 antennae and 2 pairs of eyes; the peristomium, where the mouth opening lies, possesses 1-2 pairs of peristomial cirri; the parapodia are usually uniramous, with dorsal and ventral cirri, and the bundle of chaetae, although a biramous condition is developed during the epitokous phase.

One of the easiest way to recognize a syllid is the presence of the proventricle, a muscular barrel-shaped structure associated with the pharynx, usually visible by the translucent body wall. Taxonomy of the group is based mainly on the dentition patterns of the pharynx, and on the morphology of the proventricle, chaetae and antennae and cirri throughout.