This paper gives an overview of the research carried out on sodium batteries in the last 50 years. The discovery of the very high Na+ ion conductivity in β-Al2O3 opened the way to high-energy batteries (sodium/sulfur and sodium/NiCl2) for load leveling and electrical vehicles. Then, the liquid electrolyte batteries were considered with intercalation-based electrodes and lithium and sodium as mobile ions. When Sony proposed the lithium-ion batteries, most research moved to lithium systems, exhibiting a higher energy density. These are now currently used in electronic devices, hybrid electrical vehicle (HEV) and electrical vehicle (EV). The development of renewable energies, which have an intermittent character, requires very large batteries for frequency regulation and peak production shift. For these stationary applications the most important parameters are the lifetime, the power, the price, and the material availability. Since sodium ion batteries are able to satisfy these criteria, these new material families now concern a large part of the scientific community. This article gives a general introduction and an overview of the research carried out on the materials that are presented in detail, in the articles within this special issue on sodium batteries.Read less <