Research shows a decline in physical activity levels during adolescence, particularly among girls. This study explored perceived barriers to participation in moderate and vigorous physical activity among adolescent girls who live in a large ethnoracially and socioeconomically diverse city. A total of 73 adolescent girls in Toronto participated in 1 of 7 focus group sessions which were audiotaped and transcribed verbatim, and qualitative software was used to facilitate data analysis. A constant comparison approach was used to inductively develop themes. Participants' perceived barriers to participating in physical activity included: lack of time; involvement in technology-related activities; influence of peers, parents and teachers; concern about safety; inaccessibility of facilities and cost of using them; competition; and body-centered issues. The results suggest that strategies to address specific intrapersonal, social-environmental, and physical-environmental barriers to physical activity participation should be used in physical activity promotion programs for adolescent girls.