The instrument used to listen to heartbeats is a stethoscope. This medical device allows healthcare professionals, like doctors and nurses, to assess heart sounds, lung sounds, and other bodily functions. The stethoscope consists of a long flexible tube connected to a chest piece, which typically has a diaphragm and a bell. The diaphragm detects high-frequency sounds, while the bell is better suited for lower frequencies. By placing the chest piece on the patient's body, healthcare providers can listen for abnormal sounds, helping in diagnosing various medical conditions, making it an essential tool in clinical practice.