This article gives an overview of available state-of-charge (SoC)-indication methodologies and highlights many related practical implementation issues. First of all, a brief history of the developments in SoC indication is given. The lithium-ion battery technology is briefly discussed as illustration, because battery behavior needs to be understood and taken into account properly to achieve a desired level of accuracy. The focus is on describing the background of behavior that is important to be addressed by the SoC-indication system, such as aging processes. The available SoC-indication methods are classified and advantages and disadvantages are listed. Some examples of practical implementations are given, including important issues to address. As an illustration, a state-of-the-art SoC-indication system for lithium-ion batteries is described at the end of the article.