We agree that vulnerable persons should be afforded additional attention or protection. Attempts to define who is vulnerable and what protections are required have, however, led to multiple accounts which often fail to clarify the issue in practice. This article reviews the different versions of vulnerability found in the literature. It then illustrates, through the example of children, how the application of a version devised for application in research and healthcare serves to clarify the identification and protection of vulnerable persons. In this approach, protecting the vulnerable requires a diagnostic approach. It requires that we identify morally protected interest, situations where they are fragile, the reasons why, and those involved in a duty of protection. In the case of children, the fact that parents are expected to act as their child’s protector adds a step to this analysis. Any circumstance that makes parents less likely to be either act as protector or to be effective in protecting a child will compound the vulnerability of this child.