The use of wind and solar energy to carry out the electrochemical splitting of water is a subject of considerable contemporary interest, since hydrogen produced by this means could be used to store renewable but intermittent electrical energy, to power fuel-cell vehicles, and to hydrodeoxygenate biomass or reduce CO₂ to carbon-containing fuels.^1–5  The challenge, therefore, is to develop energy-efficient systems utilizing renewable sources of electricity to achieve the electrochemical splitting of water. This chapter reviews the catalyst requirements for achieving this goal using earth-abundant elements. Particular attention is devoted to oxygen evolution, which occurs at the electrolyzer anode, since this reaction requires a potential well above the minimum set by thermodynamics. We begin by summarizing the thermodynamics for the electrochemical splitting of water and then review what is known about catalysts based on earth-abundant elements that can be used to promote the oxygen evolution reaction. This overview will show that the most promising catalysts are based on oxides and oxyhdroxides containing Ni and Fe. We will next examine in some detail how Fe³⁺ cations incorporate into Ni oxyhydroxides (NiOOH) and what the role of these cations is in promoting the OER. The chapter will end with an examination of recent efforts to find additive elements other than Fe that could be used to enhance the activity of NiOOH and elements that might be added to Ni_1−xFe_xOOH to further enhance its activity.