Light continua extending from the i.r. to the u.v. are generated with picosecond laser pulses in liquids and solids. It is demonstrated that the light spectra in water are produced by stimulated parametric fourphoton interactions, which are resonantly enhanced by single and difference frequency resonances of the non-linear susceptibility . A theory of the various parametric four-photon interaction processes is developed and the resonant structure of is discussed. The experimental results are compared with theoretical calculations. The generation of light continua by other nonlinear optical processes is briefly discussed.