Electrolytes are indispensable components in electrochemistry and the fast-growing electrochemical energy storage markets. Research in electrolytes has witnessed exponential growth in recent years, accompanied by their applications in the most popular electrochemical cell ever invented, lithium-ion batteries (LIBs). In myriads of LIBs, electrolytes and their interphases determine how high the voltage of a battery is, how many times it can be charged/discharged, or how rapid the energy stored therein could be released. The conquest of further technical challenges around safety, life and cost-effectiveness of lithium-based or beyond-lithium batteries requires in-depth understanding of electrolytes and interphases. This will be the authoritative textbook for those entering the field. Chapters will establish the fundamental principles for the field, before moving onto important knowledge acquired in recent years. There will be special emphasis on linking these fundamentals to real-world problems encountered in devices, especially lithium-ion batteries. The book will be suitable for advanced undergraduate and postgraduate students in electrochemical energy storage, electrochemistry, materials science and engineering, as well as researchers new to the subject.