This study examined children’s and adolescents’ selection of sources of self-efficacy in physical education and sport, and age-related differences in the sources. Twenty-four children, ages 8 to 14 years, were individually interviewed to learn which sources they utilized to form their beliefs that they could be successful during physical education and sport. Using an inductive content analysis (Patton, 1990), the data were separated into three age groups: 8 to 9 years, 10 to 12 years, and 13 to 14 years. Results indicated that age-related differences did occur in the selection of sources of self-efficacy. Performance was an important source for all ages. However, younger children named participation and subjective measures of success important, whereas, older children named practice hard to improve, comparisons with others, and objective measures of success important. Praise and encouragement from significant others remained important for all ages, with peers and coaches becoming more important with age.