Mentoring is a highly individualized educational measure that can support youth development in communities, schools, and talent domains. Depending on the target population, goals, structure, and medium, mentoring for youths can differ considerably. This article first reviews the main types of mentoring programs and practices for youth development in communities, schools, and talent domains. Despite the popularity of mentoring programs, many programs fail to realize the full potential of mentoring as meta-analyses consistently show relatively small effects of mentoring. The discrepancy between the potential and actual effect of mentoring is referred to as the mentoring paradox. Crucial aspects that are held responsible for the mentoring paradox, such as adequate planning and implementation of mentoring programs, adherence to research-based mentoring practices, as well as quality assurance of mentoring programs through systematic program research and evaluation are described. Finally, implications on how to professionalize mentoring are provided for different stakeholders.