© 2012 Springer Science+Business Media, LLC. All rights reserved. Before discussing the neutronics, systems, and safety considerations involved in designing fast spectrum reactors, it is appropriate to follow the lead of Wirtz  in sketching the bases for fast spectrum reactor designs. In this orientation, care will be taken to indicate the principal differences relative to thermal reactor systems with which the reader is more likely familiar. This introduction to design begins with a brief discussion of major design objectives, followed by an overview of the mechanical and thermal systems designs of fast spectrum reactors (with an emphasis in this chapter on fast breeder reactors, since most other applications of fast spectrum systems-such as waste transmutation-will be optimized if the reactor has a high internal conversion ratio and/or a high breeding ratio). Because of the position occupied by the sodium-cooled fast reactor (SFR) in the international fast spectrum reactor community, that system will be used for purposes of illustration.