This article contains an overview of three decades of research, theory development, and clinical application about ambiguous loss. Although the work includes both physical and psychological types of ambiguous loss, the focus is the aftermath of 9/11 (September 11, 2001), when the World Trade Center collapsed following terrorist attacks. On the basis of her previous work, the author was asked to design an intervention for families of the missing. She reflects on what she learned from this unexpected test and presents new propositions and hypotheses to stimulate further research and theory that is more inclusive of diversity. She suggests that scholars should focus more on universal family experience. Ambiguous loss is just one example. Encouraging researchers and practitioners to collaborate in theory development, she concludes that research-based theory is essential to inform interventions in unexpected times of terror, and in everyday life.