Mentoring programs attempt to foster a relationship that is too often missing from the lives of disadvantaged children and youth. However, in view of both the power and the limitations of mentoring programs, it is important to understand how mentoring occurs naturally. Assuming an ecological perspective, we examine mentoring in four contexts: classrooms, youth development organizations, work and service-learning, and faith- based organizations. Although none has been studied in sufficient depth, and no research has been found on natural mentoring in faith-based organizations, available evidence indicates that such mentoring adds to young people's intellectual, psychological and emotional, social, and, to a lesser extent, physical assets. This review poses many questions for future research, including fundamental ones about the prevalence of mentoring in these contexts, the circumstances in which it arises, and its risks and benefits. Nonetheless, the evidence is sufficient to encourage practitioners to promote mentoring relationships of youth and adults in all of these contexts. © 2006 Wiley Periodicals, Inc.