Three Phase Solid State Relays (SSRs) have been available since shortly after the introduction of Single Phase SSRs in 1970. The first of these relays were nothing more than 3 individual SSRs with a common input connection packaged in a single housing. This basic design concept remains, for the most part, unchanged today. However, even though the function of single phase and three phase relays are quite similar, their applications can vary significantly. This is primarily due to the characteristics of three phase power circuits and the attributes / demands of three phase loads. The proper selection and application of three phase SSRs for Heater control applications depends not only upon a comprehensive understanding of three phase SSRs, but also of the three phase power system and typical loads as well. This paper includes a summary review of three phase power systems, the characteristics of common three phase heating loads, and a comprehensive discussion of three phase SSRs and their application relative to such loads.