Dielectric elastomers, or more precisely dielectric polymers, are a relatively new class of materials for variable-stiffness and variable-damping devices. Dielectric elastomers offer significant potential advantages compared with previous variable stiffness smart materials. Advantages include simplicity, broad dynamic range, ability to reach zero stiffness, low cost, minimal mass, shock tolerance, simple drive circuitry, and design flexibility. Dielectric elastomer devices in the variable-stiffness mode convert electrical and mechanical energy back and forth to change mechanical impedance. This type of electromechanical transduction distinguishes the variable stiffness mode from simply using a dielectric elastomer actuator to alter the geometry, and hence the stiffness, of the device. Stiffness using a variable-stiffness mode depends heavily on the electrical loading of the dielectric elastomer. Analysis shows that a constant voltage loading reduces stiffness, down to zero in some cases, while a constant charge loading increases stiffness. Many applications can be considered for dielectric elastomer variable-stiffness mode such as vibration control, suspensions, and tuning consumer devices for user comfort or preference. As with dielectric elastomer actuators and generators, silicones and acrylics are the most promising materials for variable-stiffness-mode devices.