Electrorheological fluids (ER fluids) are materials with the potential to radically change the way electromechanical devices are designed and operated. Working from constitutive equations of the basic ER fluid phenomena, equations for assessing the feasibility of using ER fluids in clutches, dampers, and mounts can be derived. Using these equations, the impracticality of using currently available ER fluids in primary suspension dampers and power train clutches can be clearly demonstrated. The practicality of using current ER fluids in engine mounts can also be demonstrated. Finally, the feasibility assessments of the clutch and dampers can be used to determine the improvements necessary in the performance of ER fluids which will make these applications feasible.