A Holter monitor is a small electronic device about the size of a flip phone that keeps track of your heart rhythm, typically for one or sometimes two days. During that time, wires attached to your skin transmit your heartbeats to the device, which records all of them. It is usually performed after a traditional EKG (electrocardiogram) if the EKG doesn’t give your doctor enough information about your heart’s condition. If Holter monitoring doesn’t capture your heartbeat (concern) SYMPTOMS, your doctor may suggest an event monitor, which can work for weeks. While wearing a Holter monitor may be a little inconvenient (it’s not painful or invasive), it’s an important test that may help your doctor diagnose your condition.