Over the past decade, multijunction (MJ) solar cell technology for terrestrial concentrator applications has experienced a rapid surge in demonstrated efficiency [1-5], as illustrated in Fig. 10.1. Multijunction cells have reached the point at which the next set of technology improvements are likely to push efficiencies over 42%. Spectrolab's roadmap for terrestrial concentrator solar cell technologies with ever-increasing cell efficiency is shown in Fig. 10.2. Very high solar cell efficiencies are crucial to the cost-effective commercialization of concentrator and flat-plate photovoltaic systems alike [6-9], because of the highly leveraging effect that efficiency has on module packaging and balance-of-system costs. In this chapter, we discuss the semiconductor device research paths being investigated with the aim of reaching > 40% efficiency, and packaging, systems and economic considerations for cost-effective concentrator photovoltaics.