This paper presents a review of supplemental damping devices used for the control of the seismic response of structures. The mechanical properties of these devices are discussed and considerations in the design of energy absorbing systems are presented. Conventional structures passively resist earthquakes through a combination of strength, deformability and energy absorption. They have very little damping, so elastic energy absorption is small. Strong earthquakes deform these structures well beyond their elastic limit through localized plastic hinging, which results in increased flexibility and energy dissipation. Most of the earthquake energy is absorbed by the structure through localized damage of the lateral force resisting system. This is somewhat of a paradox in that the effects of earthquakes (i.e. structural damage) are counteracted by allowing structural damage. Structural performance can be greatly improved if a large portion of the input energy can be absorbed, not by the structure itself, but by some type of supplemental device. This paper describes a number of ways to do this, including friction devices, yielding metal systems, elastomeric viscoelastic dampers and fluid viscous dampers.