As shown in Table 2.3, solar photovoltaics has to be deployed at a scale of tens of peak terawatts or it will make little impact on our future energy mix. Terawatt-scale deployment of any of the cell technologies in Fig. 2.1 requires massive amounts of natural resources such as electricity and raw materials. The limited supplies of these resources will likely prevent many of them from reaching terawatt scales. This chapter begins with a discussion on requirements for a terawatt-scale low-cost high-efficiency solar cell technology.