This book focuses on ligand-protected gold clusters featuring their atomically precise compositions and unambiguous structures. They mimic electronic shell structures of atoms and are called structurally defined superatoms. The book describes the design principle of superatomic electronic structures and the bonding theory of superatoms as revealed by gas-phase anion photoelectron spectroscopy, which is conducted by using a state-of-the-art home-built apparatus and sheds light on fundamental electronic structures such as density of states and electron affinities otherwise elusive. This study revealed that the energy level of superatomic orbitals can be tuned coarsely by the heterometal doping and finely by the stepwise ligand exchange, respectively. The bonding theory of superatoms was also developed by anion photoelectron spectroscopy of homonuclear and heteronuclear superatoms. The comprehensive review of superatoms and detailed explanation of the apparatus were described in addition to individual studies. This book provides design principles of structurally defined superatoms and stimulates future research on the chemical and physical properties of superatoms.