Bed bugs have small, flat, oval-shaped bodies. They are wingless. Adults do have the vestiges of wings called wing pads, but they do not fully develop into functional wings.

Adults are brown in color, although their bodies redden after feeding. Full-grown bed bugs move relatively slowly and measure between 4 to 5 mm. Homeowners sometimes have the misconception that bed bugs are too small to see with the naked eye. The nymphs may be small and difficult to see, but the adults are detectable with the naked eye and may be found in the cracks and crevices they use to hide.

Newly hatched nymphs are approximately the size of the head of a pin and are white or tan until they feed. They often are described as being about the size and shape of an apple seed.