This paper describes the development of the "Chicago Children's Supervision Taxonomy" which operationally defines supervision based on the age of an injured child and the ages, familiarity, and proximity of that child's companions. The reliability, coverage, and utility of this taxonomy are illustrated by its application to 142 cases of urban childhood pedestrian injury. All cases were unambiguously classified with good interrater reliability. Most injured children were in unsupervised groups (42%) but 36% had supervisors nearby, thus, supervisor presence does not guarantee protection. Supervising more than one child (especially likely when the supervisor was a teenager) may increase injury risk compared with one-to-one supervision. The taxonomy provides a needed framework adaptable for describing direct supervision in most child injury situations and can facilitate studies of more complex aspects of supervision.