We describe two different strategies for electrical modulation of terahertz radiation. The first relies on active metamaterial arrays, while the second uses the extraordinary optical transmission through a sub-wavelength plasmonic structure. In both cases, the transmission is switched by using an external bias to control the free electron carrier density in the substrate. These modulators are characterized by high contrast, high speed, and low cross-talk, which enables applications in real-time terahertz imaging.