Silicon-based high speed electro-optic modulators are essential components of high-capacity wavelength-division-multiplexed transmission systems of the present and next generations. They greatly increase the bandwidth and speed of fiber transmission systems at a low cost. Currently, electro-optic modulators with switching speeds exceeding 6GHz have been fabricated using methods compatible with CMOS manufacturing processes. We describe various kinds of silicon-based electro-optic modulators based on different electrical and optical structures, and discuss the advantages and disadvantages in their fabrication and application. The latest progress in the development of such modulators is summarized.