  We present new observational results on the RR Lyrae K-band Period-Luminosity
relation (PLK). Data on the Galactic globular clusters NGC 3201 and NGC 4590
(M68), and on the Large Magellanic Cloud cluster Reticulum are shown. We
compare the observed slopes of the PLK relations for these three clusters with
those predicted by pulsational and evolutionary models, finding a fair
agreement. Trusting on this finding we decided to adopt these theoretical
calibrations to estimate the distance to the target clusters,finding a good
agreement with optical-based RR Lyrae distances, but with a smaller formal
scatter.
