The aim of this study is to provide an overview of the physics principles ruling optical modulation in Si and the efforts made in the last 25 years to fabricate Si-based light modulators. In the first part the fundamental parameters to describe a modulator are defined, and both the mechanisms for light modulation and the kind of device that can be fabricated are discussed. The second part is devoted to provide an overview of the devices proposed so far in literature. Of course, just a few examples are presented. The different modulator working principles are described and the structures limits evidenced. Finally, a bipolar mode field effect transistor will be taken as an example to describe how to make and, more importantly, also characterise, an electro-optical modulation in a nonconventional way.