Effective mentors are critical in the development of undergraduate and graduate students in science, technology, engineering, mathematics, and medicine (STEMM)—especially for many members of underrepresented and marginalized populations. The Science of Effective Mentoring in STEMM committee systematically compiled and analyzed current research on the characteristics, competencies, and behaviors of effective mentors and mentees in STEMM and developed a practical resource guide for mentoring practitioners to create and support viable, sustainable mentoring support systems.