Promising applications in many diverse areas of human endeavor, including medicine, communications, security, and so on, terahertz (THz) technology has recently turned into a very active area of scientific research. The THz frequency band, usually defined in the 0.3-30 THz range, was for decades one of the least explored regions of the electromagnetic spectrum, mainly due to the lack of materials and devices responding to these frequencies in a controllable manner. This work presents a study of tunable two-dimensional electron-gas (2DEG) systems and how their unique physical properties can be harnessed to develop novel high-performance active THz devices and systems.  First, a new class of highly efficient THz reconfigurable devices based on graphene is proposed and experimentally demonstrated. By employing graphene, an intrinsically 2D semiconductor as the active material, device design with unprecedented degrees of freedom, low-cost, and ease of fabrication is possible thus leading to a substantial improvement with respect to the existing art in terms of controllability of THz waves. Although in the infrared/visible range the optical absorption of graphene is only a few percent and scarcely controllable, its optical conductivity dramatically increases at THz leading to the possibility of electrical control of THz absorption. Moreover, by combining graphene layers with other passive structures augmenting the electric field intensity in the graphene, the control over THz waves can be greatly enhanced. By employing this approach THz electro-absorption modulators exhibiting better modulation-depth versus insertion-loss tradeoff than the prior art are demonstrated. These devices can be employed as the building blocks for novel THz systems; for instance a prototype single-detector THz camera was developed employing graphene electro-absorption modulator arrays. But 2DEG systems exhibit further interesting properties which may be exploitable in the THz range such as collective electron transport, i.e. electron-plasma waves, whose group-velocity can be more than one order of magnitude larger than the electron-drift velocity. Based on this phenomenon, novel device concepts for THz detectors and amplifiers are proposed. These devices, named RTD-gated plasma wave HEMTs promise operation at frequencies >> 1 THz, which has been shown to be very difficult to obtain in conventional high-speed transistors.