Abstract Attachment is deeply important to human life. When one person becomes ‘attached’ to another, their sense of security turns on their emotional, social, and physical engagement with that person. This kind of security-based attachment has been extensively studied in psychology. Yet attachment theory (in the specific sense studied by psychologists) has not received adequate attention in analytic theories of social justice. In this paper, we conceptualize attachment’s nature and value, addressing when and why attachments place justice-based claims on individuals and institutions, in an attempt to establish the centrality of attachment theory to liberal political philosophy. We first characterize security-based attachment and differentiate it from related phenomena (§1). We then explore its value, theorizing the connection between attachment, care, and companionship, drawing on the ethics of care (§2). We explain when and why security-based attachment generates claims of justice within liberal theory, noting some important difficulties (§3). Finally, we sketch some implications in three domains: the rights of those who have suffered pregnancy loss, the rights of grandparents vis-à-vis grandchildren, and the rights of attached friends to social and political recognition (§4).