Heat Sinks are required to insure the proper operation and long term reliability of Solid State Relays because they provide a means to dissipate the power that is normally developed by the SSR into the surrounding ambient air and maintain a safe operating temperature. Selecting the correct Heat Sink for any given SSR application involves coordinating form factor, size, mounting and thermal impedance rating. This paper discusses “Why Heat Sinks are Required for Reliable Solid State Relay Operation”, how the minimum required Heat Sink thermal impedance rating is calculated based upon application operating conditions, and includes an example calculation.