While the importance of having self-esteem is widely recognized and has been studied extensively, another core component of the self-concept has been relatively neglected--a sense of mattering to other people. In the current article, it is argued that mattering is an entirely unique and complex psychological construct with great public appeal and applied significance. The various ways of assessing mattering are reviewed and evidence is summarized, indicating that mattering is a vital construct in that deficits in mattering are linked with consequential outcomes at the individual level (i.e., depression and suicidal tendencies), the relationship level (i.e., relationship discord and dissolution), and the societal level (i.e., delinquency and violence). Contemporary research is described which shows that mattering typically predicts unique variance in key outcomes beyond other predictor variables. Mattering is discussed as double-edged in that mattering is highly protective but feelings of not mattering are deleterious, especially among people who have been marginalized and mistreated. The article concludes with an extended discussion of key directions for future research and an overview of the articles in this special issue. It is argued that a complete view of the self and personal identity will only emerge after we significantly expand the scope of inquiry on the psychology of mattering.