This thesis describes a method for terahertz (THz) modulation using active metamaterials. At optical and telecom frequencies, modulation of freely propagating and guided signals is an important concept that is fundamental to key technologies and applications such as communications and imaging. At THz frequencies, the ability to modulate these signals at the capacity necessary to realize these applications effectively does not exist. In order to remedy this, we investigate the use of metamaterials to modulate free space propagating waves at THz frequencies. Metamaterials are an ideal modulation platform for use in the THz frequency range because they avoid many issues that have challenged traditional THz modulators. Furthermore, the ability of metamaterials to function as artificial media holds considerable potential in the THz frequency range because it allows for a wide range of tunable material parameters extending beyond natural THz material responses. We develop and demonstrate a switchable diffractive modulator using an electrically driven metamaterial to tune transmission in real-time via voltage application. The metamaterial elements are grouped giving rise to the grating structure. We observe that the device operates as a relatively high-speed, wide-bandwidth, high-contrast modulator, with more than 20 dB of dynamic range.