Although the idea that early life conditions shape mortality is not new, there has been a resurgence of studies on the topic in the last two decades. In our review of this work, we weigh the evidence for the major causal mechanisms, i.e., biological imprint and pathway processes, thought to underlie the associations between childhood conditions and adult mortality. We also examine the evidence on how key social and physical exposures in early life are associated with adult mortality through these mechanisms. Drawing on the Health and Retirement Study, we then offer additional evidence on these associations and mechanisms. While our results reinforce the importance of early life conditions for understanding adult mortality, they, as well as previous literature, also highlight key conceptual and analytical challenges. Associations and mechanisms are sensitive, for example, to particular social and physical exposures, the period of manifestation, and the type of health problem. Nonetheless, the body of evidence strongly points to childhood conditions having an enduring influence on adult mortality through both imprint and pathway processes. We argue that variability in these associations and mechanisms should be expected both historically and within populations, as variability reflects sociohistorical differences in life-course exposures.