The last 10 years established the beginning of a post-lithium era in the field of energy storage, with the renaissance of Na-ion batteries (NIBs) as alternative for Li-based systems. The development of this technology has required intense work in materials research in order to produce and optimize anodes, cathodes, and electrolytes for NIBs. The strong and weak points of the main families of compounds for each battery component are analyzed in this progress report. Taking into account the achievements made in materials for NIBs, the industrial scene is analyzed through the existing prototypes and commercial cells and also from an economical viewpoint. In this scenario, where Na-ion technology seems to be ready for a coming second generation, the use of Na can be extended to almost the whole spectrum of electrochemical energy storage systems: the new room temperature Na–S systems, high-energy Na–air technology, or high-power Na-based hybrid supercapacitors. Thus, the degree of development of NIBs, together with the promising performance of newer Na-based energy storage systems, makes Na the key to the coming commercial post-lithium systems.