This book offers a comprehensive overview of atomically precise electrocatalysts, including single-atom, dual-atom, and multi-atom catalysts, which are considered to be superior electrode materials for fuel cells and water electrolyzers. By presenting a systematic examination of these materials in ascending order of metal atom number, the book provides a deep understanding of their synthesis processes, energy applications, and potential for improving their performance. Unlike any contemporary book on the topic, this book explores the reaction mechanisms and structure-performance relationships in catalytic processes at atomic level. Essentially, by driving the development of fuel cells and water electrocatalyzers, this book helps meet the world's growing energy demands. With its cutting-edge insights, this book is an indispensable resource for researchers, engineers, and students working in the field of renewable energy.