This book, the first of its kind, summarizes the psychological research on the concept of mattering, and outcomes associated with the presence or absence of feelings of being valued to others. Mattering, the felt experience of being valued and necessary to others, is a unique and complex psychological construct. Individual differences in mattering predict consequential outcomes at the individual, relationship, and societal levels. With a strong evidence-based focus, Gordon Flett offers unique insights from the psychological literature that draw clear links between a lack of mattering and measures of depression, anxiety, suicide risk, aggression and violence, as well as positive outcomes related to strong family relationships and measures of resilience, adaptability, motivation, and performance. Crucial links between social marginalization and mattering are explored, along with the relationship between mattering and child development. Clinical applications summarize mental health interventions to measure and address issues related to mattering in family, school, and community contexts.