 This article provides a comprehensive overview of the work to date on the two-step solar H2O and or CO2 splitting thermochemical cycles with ZN slash ZNO redox reactions to produce H2 and CO, synthesis gas. An outline of the underlying science and the technological advances in solar reactor engineering is provided along with life cycle and economic analyses. This article was authored by Aldo Steinfeld, Anton Meyer, and Peter G. Lautzenheiser.