In electrical circuits, ringing is an unwanted oscillation of a voltage or current. It is caused when an electrical pulse causes the parasitic capacitances and inductances in the circuit (i.e. those that are not part of the design, but just by-products of the materials used to construct the circuit) to resonate at their characteristic frequency. Ringing artifacts are also present in square waves.

Ringing is undesirable because it causes extra current to flow, thereby wasting energy and causing extra heating of the components; it can cause unwanted electromagnetic radiation to be emitted; and it may cause unwanted triggering of bistable elements in digital circuits.

In hearing ringing is an endaural phenomenon in which a person hears a sound, somewhat like a sine tone , without any external acoustic stimulation. Ringing in the ears commonly follows exposure to loud noises, and is a sign of damage to the hair cells of the inner ear. Ringing in the ears is a common sort of tinnitus.