Compared to the neighboring infrared and microwave regions, the terahertz regime is still in need of fundamental technological advances. This derives, in part, from a paucity of naturally occurring materials with useful electronic or photonic properties at terahertz frequencies. This results in formidable challenges for creating the components needed for generating, detecting, and manipulating THz waves. Considering the promising applications of THz radiation, it is important overcome such material limitations by searching for new materials, or by constructing artificial materials with a desired electromagnetic response. Metamaterials are a new type of artificial composite with electromagnetic properties that derive from their sub-wavelength structure. The potential of metamaterials for THz radiation originates from a resonant electromagnetic response which can be tailored for specific applications. Metamaterials thus offer a route towards helping to fill the so-called "THz gap". In this work we discuss novel planar THz metamaterials. Importantly, the dependence of the resonant response on the supporting substrate enables the creation of active THz metamaterials. We show that the resonant response can be efficiently controlled using optical or electrical approaches. This has resulted in the creation of efficient THz switches and modulators of potential importance for advancing numerous real world THz applications.