This book introduces readers to the preparation of metal nanocrystals and its applications. In this book, an important point highlighted is how to design noble metal nanocrystals at the atomic scale for energy conversion and storage. It also focuses on the controllable synthesis of water splitting electrode materials including anodic oxygen evolution reaction (OER) and cathode hydrogen evolution reaction (HER) at the atomic level by defect engineering and synergistic effect. In addition, in-situ technologies and theoretical calculations are utilized to reveal the catalytic mechanisms of catalysts under realistic operating condition. The findings presented not only enrich research in the nano-field, but also support the promotion of national and international cooperation.