This poster presented results from a detailed analysis of observed and theoretical light-curves of classical Cepheid variables in the Galaxy and the Magellanic Clouds. The theoretical light-curves were based on non-linear convective hydrodynamical pulsation models; the observational data were taken from ongoing wide-field variability surveys. The variation which we found in theoretical and observed light-curve parameters as a function of period, wavelength and metallicity was used to constrain the input physics to the pulsation models, such as the mass–luminosity relations obeyed by Cepheid variables. We also accounted for the variation in the convective efficiency as entered into the stellar pulsation models and its impact on the theoretical amplitudes and Period-Luminosity relations for Cepheid variables.