Since the late 1980s, feminist scholars across the sciences and the humanities have argued that sex, in addition to gender, was socially constructed in at least two senses of the term. First, our understanding of sex, itself –that is, our understanding of biological sex and its multiple material components – is shaped by socio-culturally and temporally specific meanings, and material arrangements. Second, the sexed body itself is sociomaterially constituted insofar as its development is inextricably entangled with the larger social and material environment where it occurs. Thus, while we may find evidence of sex differences within and across bodies, those differences may be better characterized as material effects of development within a gendered environment than evidence of naturally binary sex. This chapter explores the social construction of sex in both of these senses, while highlighting the fuzzy and pervious nature of the distinction between them, due to the dynamic, contextually dependent nature of both the development of sex and our accounts of it. Finally, this chapter turns to consider emerging feminist engagements with the social construction of sex across a variety of disciplines that have important implications for feminist philosophers of science. In particular, I highlight work that seeks to refine our analyses of sex and gender as temporal (and temporally) constituted objects, and to further elaborate the co-construction of sex with other axes of identity – particularly race and disability.