Population ecology is concerned with measuring changes in population size and composition, and identifying the causes of these fluctuations. Important driving variables include animal body size and growth rate, and their relationship to reproduction and mortality. Among wild and domestic reindeer (Rangifer tarandus tarandus), reproductive performance, calving time, calf birth weight and neonatal mortality are strongly correlated to maternal weight. Heavy females enjoy higher pregnancy rates, calve earlier, and give birth to heavier calves which have a higher neonatal survival rate than light females. Most studies indicate that both weaning weight of a calf and mature body weight correlate to its birth weight. Calf body weight and composition influence the rate of attainment of sexual maturity. Females which breed as calves suffer reduced growth and give birth to smaller calves, which suffer higher neonatal mortality and lower rates of postnatal growth. A yet unresolved question is whether reindeer body weight, and hence reproductive performance and neonatal mortality, are more strongly influenced by winter than by summer grazing conditions. This paper reviews population ecology studies on wild and domestic reindeer and promotes the view that body size in Rangifer is determined primarily by grazing conditions during the summer.