1. Introduction

Solar photoelectrochemical (PEC) hydrogen production is one of the promising technologies that could potentially provide a clean, cost-effective, and domestically produced energy carrier by taking advantage of the *120,000 TW of radiation that continually strikes the earth’s surface. To date, no cost-effective materials system satisfies all of the technical requirements listed above for practical hydrogen production. Despite the challenges, there are promising pathways for achieving the important goal of efficient, cost-effective PEC hydrogen production. For continued progress in overcoming the most important remaining scientific and engineering barriers, widely accepted standards in the characterization and reporting of PEC materials and devices are needed.