Sodium-ion batteries are likely to be the next-generation power sources. They offer higher safety than lithium-ion batteries and, most important, sodium is available in unlimited abundance. The book covers the fundamental principles and applications of sodium-ion batteries and reports experimental work on the use of electrolytes and different electrode materials, such as silicon, carbon, conducting polymers, and Mn- and Sn-based materials. Also discussed are state-of-the-art, future prospects and challenges in sodium-ion battery technology. Keywords: Sodium-Ion Batteries, Lithium-Ion Batteries, Carbon Nanofibers, Conducting Polymers, Electrode Materials, Electrolytes, Graphene, Carbon Anodes, Magnetic Nanomaterials, Mn-based Materials, Sn-based Materials, Na-O2 Batteries, NASICON Electrodes, Organic Electrodes, Polyacetylene, Polyaniline, Polyphenylene, Redox Mediators, Reversible Capacity, Singlet Oxygen, Superoxide Stability.