The main objective of the presented research is to design, fabricate, fully characterize, and assess the usability and functionality of a novel wireless tongue-operated assistive technology, called Tongue Drive System (TDS), that allows individuals with severe physical disabilities (such as quadriplegics) to effectively access computers, drive powered wheelchairs, and control environments using their voluntary tongue motion. The system can wirelessly detect users' tongue movements using an array of magnetic sensors, and a magnetic tracer secured on the tongue, and translate them into a set of user-defined commands in real time, which can then be used to communicate with target devices in users' environment. The principal advantage of the TDS is that a combination of magnetic sensors and a small permanent magnet can capture a large number of tongue movements, each of which can represent one specific command. A set of dedicated tongue movements can be configured as specific commands for each individual user based on his/her preferences, lifestyle, and remaining abilities. As a result, this technology can benefit a wide range of potential users with different types of disabilities. The work carried out in this dissertation is largely split into three portions: (1) Development, fabrication and characterization of external TDS (eTDS) prototypes to verify the concept of TDS that is detecting and extracting user's intention through their voluntary tongue motion, utilizing a combination of magnetic sensors and a small magnet, as well as the application of this idea in the context of assistive technology. This part of the work is presented in Chapters IV, V and VI. (2) Assessment of the TDS performance in medium term usage for both computer access and wheelchair control. The main purpose of this work was to gain valuable insight into the TDS learning process and its current limiting factors, which could lead the way in designing new generations of TDS with improved usability. This portion of the work is described in Chapter VII. (3) Development and performance assessment of a multimodal TDS (mTDS), that operates based on the information collected from two independent input channels: the tongue motion and speech. This multimodal system expands the access beyond one input channel and therefore improves the speed of access by increasing the information transfer bandwidth between users and computers. This part of the work is presented in Chapters VIII and IX. This dissertation has contributed to the innovation and advancement of the start-of-the-art assistive technology research by exploring, realizing and validating the use of tongue motion as a voluntary motor output to substitute some of the lost arm and hand functions in people with severe disabilities for computer access, wheelchair navigation, and environmental control.