Experimental data on the nonlinear optical molecular hyperpolarizabilities of push-pull polyenes (measured with the vibrational method recently proposed) are presented and discussed. The analysis of the results and the interpretation of the spectral features allow us to establish structure-property relationships. These relationships are particularly helpful in the understanding of the mechanisms which rule the nonlinear optical behavior of organic polyconjugated compounds and in view of the design of new and improved materials. Comparison with other techniques, when available, correlate nicely with the conclusions reached with the vibrational method.