  We present a unified description of the resonance peak and low-energy
incommensurate response observed in high-$T_c$ cuprate superconductors. We
argue that both features have a purely magnetic origin and they represent
universal features of an incommensurate spin state both below and above the
superconducting transition temperature. In this description the resonance peak
is the reflection of commensurate antiferromagnetism. Our theoretical scenario
gives an account of the main features observed in various families of
superconductors and predicts those not yet observed, like a resonance peak in
La$_2$NiO$_{4+x}$.
