Electroheological (ER) fluids have electrically controllable stiffness, heat transfer and flow properties. Since their invention in 1947, they have been proposed for a variety of applications involving the electrical control of systems such as hydraulic valves, clutches, heat exchangers and suspension systems. Previous approaches to application of ER fluids have been hampered by the relatively slow, strongly time-dependent, non-linear behavior of these fluid systems. The effects of electric field activation history, temperature and humidity also contribute to wide variation in 'open-loop' sped and strength of response. Successful application of ER fluids to engineering systems requires fast, precise control of the internal micro-scale fluid state which yields the controllable macro-scale properties to be exploited. This work presents a 'closed-loop', laser- sensing, feedback control approach of ER fluid state which allows for higher initial field strengths to speed ER response while lowering the level of applied electric field to exactly that level required to maintain a specified level of ER fluid viscosity, stiffness, thermal conductivity or radiative energy transmissibility. The key to the work is a laser-based optical sensor of fluid internal state. An analytical model for both the ER fluid and control systems are developed which predicts ER fluid system response as controlled field drive is varied. Predicted ER fluid responses from the analytical model are then compared with laboratory measured responses for a prototype feedback controlled ER fluid system. Laser sensing and feedback allows the us of these fluids in a wide variety of applications where the lack of fast, precise control limited their past use. The ability to quickly and precisely control ER fluid response may make possible the applications of ER fluids promised since their invention 5 decades ago. When compared against conventional 'open-loop' fluid control methods, laboratory tests of 'closed-loop' feedback control demonstrate ER fluid response both 30 times faster with 30 times more precision than previously possible.