Insects can sense a broad range of acoustic signals transmitted through air, water, or solids. Sounds and vibrations are used for orientation, detection of predators and parasitoids, and sexual and social interactions. There is tremendous morphological diversity of receptor organs, ranging from single hairs that detect near-field sounds to complex tympanal ears with thousands of sensory cells. Hearing organs can occur on almost any body part, including the legs, mouthparts, and wings. Despite their morphological diversity, most insect ears are associated with a thin cuticular vibrating structure, tracheal air sacs, and chordotonal sensory organs. We review acoustic receptors in different insect taxa, and discuss structural features associated with functional specializations.