Fireflies, lightningbugs, glowworms. Adults soft-bodied, elongate; color generally black, with red markings on the pronotum; antennae with 8-13 (usually 11) segments, filiform, serrate or branched in some taxa; tarsi 5-segmented, next-to-last segment with pads; abdominal segments may be modified as luminescent organs (mainly Eastern species). Some adults may have reduced wings or even be larva-like.

Larvae live in moist soils, sometimes along shorelines; all larvae luminescent to some degree; body elongate with prominent scleritized plates; head covered by pronotum; legs 5-segmented.