Standard poverty analysis makes statements about deprivation after the veil of uncertainty has been lifted. Nonetheless, the term 'vulnerability' has been used as a tool to remark that uncertainty and risk do matter. In this paper, we define 'vulnerability to poverty' as the magnitude of the threat of poverty, measured ex-ante, before uncertainty is resolved. We describe the desirable properties of a vulnerability measure as a set of axioms, and present a family of measures satisfying our desiderata at the individual level. We also propose a family of measures of aggregate vulnerability, a concept which has remained largely unexplored thus far.