Immobilizing particles or droplets on electrodes is a novel and most powerful technique for studying the electrochemical reactions of three-phase systems. It gives access to a wealth of information, ranging from quantitative and phase analysis to thermodynamic and kinetic data of electrode processes. Three-phase electrodes with immobilized droplets provide information on the electrochemistry of redox liquids and of compounds dissolved in inert organic liquids. Such measurements allow the determination of the Gibbs energies of the transfer of cations and anions between immiscible solvents, and thus make it possible to assess the hydrophobicity of ions a property that is of great importance for pharmaceutical applications, biological studies, and for many fields of chemistry.The monograph gives, for the first time, a comprehensive overview of the results published in more than 300 papers over the last 15 years. The experiments are explained in detail, applications from many different fields are presented, and the theoretical basis of the systems is outlined.