Modulation of the laser using electro-optic materials has been studied. In part I, an experimental study of the modulation of He-Ne laser using a transverse KDP cell is described. Both DC and AC characteristics are measured and analyzed. The experimental results are in good agreement with the theory within the experimental error. The bandwidth of the modulator is about 2 MHz. In part II, a brief review on the electro-optic modulator is given. Characteristics of the most important materials(KDP-type crystals, $LiNbo_3$, $LiNbo_3$, and GaAs) are summarized. Then, design considerations and the various schemes for the bulk-type transverse modulators are discussed. Finally, the state of the art of the most promising waveguide modulator is described.