Water-electrolysis-based hydrogen generators have been in commercial use for more than a century. With the global drive toward renewable energy, very-large-scale electrolysis systems are being installed to convert inexpensive renewable electricity and water into commodity hydrogen for industrial, commercial, transport fuel, and energy storage applications. Along with scale-up of hydrogen production, there is growing interest in byproduct oxygen gas in electrolysis installations. Additionally, small-scale packaged electrolyzers continue to be installed to generate hydrogen on demand, minimizing or eliminating the need to store significant amounts of hydrogen on site. Throughout the past century, important learnings have become electrolysis best practices, and lessons continue to be learned as electrolysis deployments and technology evolve.