This paper addresses, starting from an extensive series of tests in longitudinal regular waves, the problems connected with the threshold formulation for parametric rolling and its amplitude modelling above threshold. Both head and following waves have been considered, also in view of the greater attention to head sea conditions called during IMO/SLF discussion on the revision of the Intact Stability Code. Particular attention is given to the nonlinear damping, nonlinear restoring and nonlinear parametric excitation terms. The mathematical models so developed are “compared” with experimental results by means of an ad hoc parameter estimation technique. The experimental results show that a preliminary analysis of the possibility of the onset of parametric rolling can be done by using standard hydrostatic packages allowing for the computation in longitudinal waves. The amplitude above threshold needs, however, a nonlinear approach. The subject is discussed in connection with a series of experiments on scale models of naval ships (a frigate and a destroyer) which exhibited large amplitude rolling in head waves in some loading conditions.