Sias and Bartoo (2007) described friendships as a psychological "vaccine" against both physical and mental illness. They hypothesized that prophylactic benefits are often derived from the emotional, tangible, and informational support provided in close personal friendships. Other clinical researchers have posited that broader forms of social support provide resiliency by "buffering" reactions to life stress (Turner & Brown, 2010). This chapter reviews evidence in support of the contention that personal friendships and social support enhance resiliency to stressors such as trauma, losses, maltreatment, and other developmental adversities. This literature review will be followed by an analysis of original data that provides a test of the general hypothesis that close child and adult relationships portend better overall mental health. Links between friendship and mental health indices are complex. First, friendship represents a complex construct without a uniform definition. Second, mental health symptom clusters extend across many relevant dimensions that vary in their sensitivity to interpersonal influences. Third, relationships between mental health and friendship variables, however measured, are inherently complicated by their bidirectional nature. While cause-effect relationships prove difficult to establish, collective correlational findings are useful in identifying the sorts of mental health symptom clusters that are most likely to emerge when critical social support and friendship circles have been destabilized. Defining Qualities of Friendship Hayes (1988) defined friendship as a voluntary interdependence of two persons over time involving companionship, intimacy, affection, and mutual assistance intended to facilitate the socioemotional goals of both parties. Sullivan (1953) emphasized decades ago that friendships serve many purposes including companionship, assistance, affection, intimacy, alliance, emotional security, and selfvalidation. Friendships also convey a sense of mutual value, enhance communication B e n e f i t s a n d M a i n t e n a n c e o f F r i e n d s h i p s 250 and interpersonal skills, and buffer both partners against life stressors (Bukowski, Hoza, & Boivin, 1994). Developmental Contributors to Friendship Capacity Secure and affirming parent-child relationships have been predictive of close and sustainable young adult friendships (Wise & King, 2008. Conversely, childhood maltreatment and other forms of developmental adversity may have deleterious effects on the capacity of the child to develop healthy friendships and other interpersonal relationships. Childhood abuse victims appear to have greater difficulty in initiating and sustaining satisfying peer relationships (Smith, 1995). Parental physical abuse has been found to predict less rewarding adult best friendships (Mugge, King, & Klophaus, 2009). Children from abusive homes have reported that they feel more negative toward a greater portion of their best friendships than do children with nonremarkable histories (Salzinger, Feldma...