This paper reviews recent work in the area of active metamaterials where transistors and circuitry are embedded within metamaterial structures for novel functions. In one function, embedding of psuedomorphic high electron mobility transistor (pHEMT) within the metamaterial resonator allows realization of a terahertz modulator. A variation of this approach utilizes diodes to modulate the metamaterial response between a perfect absorber and a perfect detector. In another function, a transistor based power detector is embedded within each metamaterial resonator for roomtemperature detection of gigahertz (GHz) radiation. The realized platform has the potential for high resolution imaging at the diffraction limit. These functions indicate range of novel devices enabled through heterogeneous integration of semiconductor devices with metamaterials.