What might clinicians make of Bowlby's first patient if he were seen by services today? How might thinking with Bowlby help us critically consider children whose presentations span a number of diagnoses, and children who do not readily fit any existing diagnostic category? We will consider these questions through exploration of Bowlby's writings, as well as review of the literature on diagnosis, trauma, and caregiving environments. We identify tensions between the three key legacies of Bowlby's work: the focus on family systems; attachment theory as a theory of individual development; and psychiatric classification of attachment. In doing so, we will use conversation with Bowlby's ideas as the basis for a consideration of the purpose and function of diagnosis.