Adult bed bugs are light brown to reddish-brown, flattened, oval-shaped, and have no hind wings. The front wings are vestigial and reduced to pad-like structures. Bed bugs have segmented abdomens with microscopic hairs that give them a banded appearance. Adults grow to 4–5 mm (0.16–0.20 in) long and 1.5–3 mm (0.059–0.118 in) wide.
Newly hatched nymphs are translucent, lighter in color, and become browner as they moult and reach maturity. A bed bug nymph of any age that has just consumed a blood meal has a bright red, translucent abdomen, fading to brown over the next several hours, and to opaque black within two days as the insect digests its meal. Bed bugs may be mistaken for other insects, such as booklice, small cockroaches, or carpet beetles; however, when warm and active, their movements are more ant-like and, like most other true bugs, they emit a characteristic disagreeable odor when crushed. Bed bugs use pheromones and kairomones to communicate regarding nesting locations, feeding, and reproduction. The lifespan of bed bugs varies by species and is also dependent on feeding.