This book describes the main aspects related to the ion transfer reactions occurring at the interface between two immiscible electrolyte solutions (ITIES). The particular topics discussed involve simple and assisted ion transfer, and chemical reactions coupled to ion transfer. The focus is on theoretical approaches, numerical simulations and experimental methodologies. Concerning the theoretical approaches, different computational simulations related to simple and assisted ion transfer are reviewed. The main conclusions drawn from the most widely accepted models are described and analysed in regard to their relevance to explain different aspects of ion transfer.