Gas reactors, military reactors, and liquid metal fast breeder reactors (LMFBRs) make up most of the rest of the power reactor installed base in the world today. In this chapter, we would like to discuss each of these designs and their relevance to the field of nuclear power as a whole. Research reactors, such as the General Atomics TRIGA reactor, are discussed in our companion book, where we will also consider the effects of temperature feedback on their behavior. *