This study explored mothers' perceptions of influences on preschoolers' physical activity. Six semistructured focus groups with 23 mothers were conducted across a range of socioeconomic position locations. Mothers identified 4 key areas of influence: child fundamentals (eg, sex, personality), parent power (eg, rules, support), people to share with (eg, peers, adults), and places and things (eg, physical environments, toys). No substantial differences in themes were identified among socioeconomic position groups. Influences on preschoolers' physical activity are multidimensional, multifactorial, and support the use of ecological models to conceptualize and understand the influencing factors. Associations among factors influencing preschoolers' physical activity should be further investigated through quantitative research.