Resonance is the tendency of a system to oscillate with greater amplitude at some frequencies than at others. Frequencies at which the response amplitude is a relative maximum are known as the system's resonance frequencies. To study the resonance in an RLC circuit, as illustrated in , we can see how the circuit behaves as a function of the frequency of the driving voltage source.

Combining Ohm's law, I_{rms}=V_{rms}/Z, and the expression for impedance Z from

where I_{rms} and V_{rms} are rms current and voltage, respectively.
The reactances vary with frequency _{L} large at high frequencies and X_{C} large at low frequencies given as:

At some intermediate frequency _{rms} results.
We can get an expression for _{L}=X_{C}.
Substituting the definitions of X_{L} and X_{C} yields:

_{rms} is a maximum.
Resonance in AC circuits is analogous to mechanical resonance, where resonance is defined as a forced oscillation (in this case, forced by the voltage source) at the natural frequency of the system.

The receiver in a radio is an RLC circuit that oscillates best at its _{rms} at