In this chapter we elaborate on our conceptual view on vulnerability being the result of insufficient reserve build-up across the life course in various domains (e.g., cognitive reserve and relational reserve). Specifically, we propose that certain individuals are more vulnerable (than other, less vulnerable individuals) to impairments in human functioning because of insufficient reserves accumulated over their life course, which consequently exposes those individuals to greater difficulty to deal with threatening situations in old age in order to offset or prevent negative outcomes. To illustrate our novel perspective with empirical examples, we focus on cognitive and relational reserve domains and their role for cognitive health and well-being as aging outcomes. First, we elaborate on the build-up of these reserve domains across the life course, with a special focus on inter-individual differences in the pathways of reserve accumulation. Second, we concentrate on the role of reserves for human functioning in different groups of vulnerable individuals in old age. Third, we elaborate on the interplay of different reserve domains and their potential joint effects on aging outcomes. Finally, we conclude with an outlook for future research.