Introduction On the occasion of my retirement from the University of Minnesota, a symposium was held to encourage the continuation of research about loss and boundary ambiguity. This special issue continues that goal. The papers herein illustrate how a new generation of scientists and practitioners applies loss theory to understand previously unstudied situations and populations. Their work generates new questions and hypotheses and, hopefully, stimulates others to join the ongoing process of research, practice, and theorizing. Why Do We Need Theorizing? In these times of crises and terror, we need new theories to guide our work in safeguarding the natural resiliency of families. To assess both diversities and commonalities in how families stay strong, we need more inclusive theory to analyze data and guide interventions for easing the family stress and trauma. I began with a universal family experience-lossand studied it in the context of an additional stressor-ambiguity. Indeed, as Dilworth-Anderson (2005) writes, intuition begins this process. Observing family therapy in the early 1970s, I noticed physically present fathers were often psychologically absent (Boss, 1972). Soon, I realized that psychological absence was not only just about fathers but also about any loved one in the family who was there, but not there. Out of observation and intuition emerged a more inclusive term, ambiguous and a model of the two types: physical absence with psychological presence, psychological absence with physical presence (Boss, 1999, 2004, 2006). Family members have described the first type of loss as Leaving without good-bye, and the second type as Good-bye without leaving. Both are distressing and may traumatize. Today, the stress- and resiliency-focused theory of loss includes linkages to meaning, mastery, ambivalence, identity, attachment, and hope (Boss, 2006). Although many practitioners report anecdotally that the theory is useful, more research-based evidence is needed. This special issue serves as a beginning for more research to continue integrating theory, research, and application. What Is Ambiguous Loss? Ambiguous loss is a loss that remains unclear. The premise of the loss theory is that uncertainty or a lack of information about the whereabouts or status of a loved one as absent or present, as dead or alive, is traumatizing for most individuals, couples, and families. The ambiguity freezes the grief process (Boss, 1999) and prevents cognition, thus blocking coping and decision-making processes. Closure is impossible. Family members have no other option but to construct their own truth about the status of the person absent in mind or body. Without information to clarify their loss, family members have no choice but to live with the paradox of absence and presence (Boss, 2006). For example, when families are separated by military deployment, they of course hope to be reunited again but also know that they will never be the same as they were before the separation. I propose that both/and thinking strengthens adults' and children's resiliency despite the ambiguity of a family member's absence or presence. That is, it is useful for a family member to think dialectically about thesis, antithesis, and synthesis in a practical way: my loved one is gone, but s/he is also here; I can learn to tolerate the stress of the ambiguity (Boss, 1999, 2004, 2006). When relationships are unclear and closure is impossible, the human need for finality can distress or traumatize families. Whether it is caring for a mate in the grip of dementia from Alzheimer's, or waiting to learn the fate of a child gone missing, the ambiguity in such losses immobilizes and traumatizes. Bereft of rituals to support them (because the loss is unverified), families are left on their own. Because of the ambiguity, relationships dissipate as friends and neighbors do not know what to do or say to families with unclear losses. …