In this chapter the authors discuss research on the effects of long and short birth spacing from the perspective of children (the time interval between the births of adjacent siblings). Recent research has placed doubt on previous findings of adverse effects of short birth intervals on birth outcomes as well as short-, medium-, and long-term health, educational, and socioeconomic attainment outcomes. They summarize recent findings with a particular focus on impacts over the lifecourse, and the extent to which findings are applicable at various levels of social and economic development. The authors also briefly discuss the effects of birth spacing in the broader context of research examining the effects of early life conditions on adult outcomes.