Kilohertz Quasi-Periodic Oscillations (QPOs) have been detected in many accreting X-ray binaries. It has been suggested that the kHz QPO frequencies observed in the modulation of the X-ray flux reflect a non-linear resonance between two modes of accreting disk oscillations. A 3:2 resonant ratio of frequencies has been clearly recognized in the black-hole QPO data supporting the non-linear resonance hypothesis. It is often discussed whether the same 3:2 ratio appears in case of the neutron star QPOs as well.