The absorptive separator plays an important role in the operation of valve-regulated lead–acid (VRLA) batteries. The composition and physical characteristics of recombinant-battery separator mats (RBSMs), also known as absorptive-glass mats (AGMs), directly affect three critical factors associated with the performance of VRLA batteries. The factors are: electrolyte supply, oxygen-transport, and constraint of positive active-material growth. This paper discusses the influence of the physical properties of RBSMs on the performance of VRLAs, explains how these properties are measured, and defines the characteristics of an ideal separator. Separators used presently in VRLAs meet some of these criteria, but development of advanced materials is required to improve battery life.