A large number of people experience neurological disorders in their life time, and these patients seek to regain their body functions with rehabilitation and assistive devices. In this dissertation, we present the development of a hand exoskeleton, called Maestro, which is designed to advance research in fields of hand rehabilitation and hand assistive devices. Maestro is mechanically and electrically robust, accurate in sensing and actuation, and compatible to various rehabilitation schemes, subjects, hardware/software, and different operators. As a result of these features, Maestro has led to research on adaptive control theories for diverse properties of hands, the development of a hand-wrist exoskeleton, the development of a novel rehabilitation framework, progress of hand muscle fatigues, and assistance for SCI patients. Particularly on the assistance for SCI patients, we present that the advantage of a compliant hand assistive device may result in high success ratios for grasping various objects required in activities of daily living (ADL) with surface EMG sensors. The hand functions of SCI subjects are evaluated with and without Maestro through a standardized hand function test called Sollerman hand function test (SHFT). The results with six SCI subjects show that the hand functions of C6 and C7 SCI subjects improved with assistance from Maestro.