In this chapter, we discuss the prerequisites for high-efficiency water splitting and their implementation with tandem cells based on absorbers of the III–V semiconductor material class. A brief outline of efficiency-limiting factors shows that at a given set of boundary conditions, such as catalyst performance, the optimum tandem absorbers require a very precise control of opto-electronic properties, as facilitated by the III–V compounds. After a short history of high efficiency solar energy conversion, we present recent implementations of highly efficient water splitting systems with solar-to-hydrogen efficiencies of 14–16% together with an outlook on further improvements. Even if other absorber systems turn out to be more cost-competitive, the III–V systems currently serve as a testbed for high-efficiency water splitting in general, with lessons to be learned for catalyst requirements, cell design, and efficiency validation. We conclude with a discussion of appropriate efficiency benchmarking routines, outlining potential pitfalls for multi-junction absorbers and how to avoid them.