In this chapter, we first review the major issues facing conventional complementary metal–oxide–semiconductor (CMOS) scaling. We then introduce the basics of a fully depleted device operation and discuss how fully depleted devices overcome the barriers that limit conventional scaling. In addition, bulk and silicon (Si) on insulator (SOI) FinFETs are compared and contrasted. The attributes of a planar fully depleted silicon on insulator (FDSOI) are reviewed and also compared with FinFETs. Finally, the ultimate fully depleted device option, a nanowire transistor, is presented and its benefits and drawbacks are shown.