Tunable electromagnetic metamaterials can be designed through the incorporation of semiconducting materials. We present theory, simulation, and experimental results of metamaterials operating at terahertz frequencies. Specific emphasis is placed on the demonstration of external control of planar arrays of metamaterials patterned on semiconducting substrates with terahertz time domain spectroscopy used to characterize device performance. Dynamical control is achieved via photoexcitation of free carriers in the substrate. Active control is achieved by creating a Schottkey diode, which enables modulation of THz Transmission by 50 percent, an order of magnitude improvement over existing devices. Because of the universality of metamaterial response over many decades of frequency, these results have implications for other regions of the electromagnetic spectrum and will undoubtedly play a key role in future demonstrations of novel high-performance devices.