Light induced changes of optical properties are in close relation with photoelectrical properties of semiconductors. From the dynamics of nonlinear optical response, not only electronic parameters of crystals but also the channels of energy relaxation, role of defects and mechanisms of light interaction with matter may be studied. Light diffraction on laser induced transient gratings has proved to be a very powerful technique for these fundamental swdies of semiconductors as well as for their applications in opto- and microelectronics. Photorefractive Ill-V and il-VI semiconducting compounds reveal optical nonlinearities of different origins under excitation by short las pulses or by CW lasers [1,2]. The contributions to refractive index modulation by free carriers and by space charge electric fields coexist at pulse excitation while solely linear electrooptic effect is responsible for optical gain in a quasi-stationary case of excitation. Carrier plasma kinetic coefficients, as ambipolar and monopolar mobiities, carrier concentrations and lifetimes, defect concentration and other parameters may be obtained from light diffraction on transient grating experiments [3-7]. In addition, at short pulse excitation a number of novel effects have been obsved: light induced space-charge (SC) field enhancement , fast oscillations of diffraction efficiency and multiexponential grating decay [9,10]. These effects probably arise from simultaneous presence of two refractive index modulation mechanisms and their interaction. Indeed, the diffusive decay of free carrier grating leads to the build-up of space charge electric fields, which in turn involve electron and hole sal separation, screening of space charge fields, and affect the decay processes of electronic gratings. A significant step towards the understanding of the temporal behavior of nonlinearities involved as well as possibilities of those optical techniques for semiconductor research may be obtained by help of a detail analysis of carrier and field dynamics in different experimental situations.