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Resonators are devices or systems that store and release energy in a periodic fashion at specific frequencies, known as resonance frequencies. When driven near these frequencies, the system can sustain oscillations by exchanging energy between stored forms, such as electric and magnetic fields or kinetic and potential energy in a mechanical structure. The rate at which oscillations decay due to losses is described by the quality factor, Q; higher Q values indicate narrower bandwidth and longer energy storage.

Resonators can be categorized by the form of energy they manipulate: electrical, mechanical, acoustic, or optical. Common types include mechanical resonators (pendulums, microelectromechanical systems), electrical resonators (LC tanks, quartz crystal resonators, dielectric resonators, cavity resonators), and optical resonators (Fabry-Pérot cavities, ring resonators, and whispering-gallery-mode resonators).

Key parameters include the resonance frequency f0, the quality factor Q, and the bandwidth Δf ≈ f0/Q. The coupling to external signals or fields determines how energy enters and leaves the resonator, affecting performance in circuits, sensors, and communication systems.

Applications span timing and frequency control, filters and oscillators in radio and microwave systems, lasers and optical communications, as well as sensing and metrology. Quartz crystal resonators provide stable timing references; dielectric and cavity resonators enable compact high-frequency filters; optical resonators determine laser wavelengths and linewidths and enable highly selective sensors.

Resonator design often involves tradeoffs among size, loss, mode structure, and environmental sensitivity, depending on the intended operating frequency range and application.