Southern elephant seal Mirounga leonine numbers declined precipitously throughout most of their circumpolar distribution since the 1950s. A long-term intensive demographic programme was initiated in 1983 on the relatively small population of southern elephant seals at sub-Antarctic Marion Island in an attempt to identify causative mechanisms associated with this decline. Weaned pups have been tagged annually since 1983, and this has produced a large number of individuals of known identity. A regular resighting programme yielded a mark-recapture dataset that has been subjected to numerous survival-based models. This ongoing programme produced a substantial body of scientific literature on population growth patterns, vital rates (survival and fecundity) and population regulation in southern elephant seals, which are reviewed in this synthesis. We briefly describe the analytical framework common to much of the demographic research, highlight important conclusions concerning population regulation of elephant seals at Marion Island, and discuss priorities for future research.