The predictions of an updated and extensive set of nonlinear, nonlocal, time dependent and convective pulsation models, spanning a wide range of masses and chemical compositions, are used to derive a theoretical metal dependent Period-Luminosity-Amplitude relation for RR Lyrae stars. Such a relation represents a very promising technique to derive the intrinsic luminosity and in turn the distance of RR Lyrae stars both in the Milky Way and in nearby galaxies. The intrinsic luminosity of RR Lyrae can be of great importance to calibrate the luminosity of the main-sequence turn-off and to constrain the age of globular clusters, where RR Lyrae are generally observed.