This article describes the functions required of the separator for the lead–acid battery to perform as designed. To ensure proper functionality of the separator, various characterization tests have been developed over the years, and these tests are reviewed. This article also reviews how developments in separators have allowed substantial improvements in battery designs, which have contributed to the expansion of lead–acid battery markets. The lead–acid battery primarily serves three markets: automotive, motive power, and stationary. These have different requirements upon the battery and also upon the separator, which are reviewed. Finally, novel developments in separator designs and how they might lead to advances in the lead–acid battery are explained.