Philosophy for Children (P4C) stems from the work of Mathew Lipman. It involves teacher modeling of exploratory talk in relation to a complex stimulus, then has students discuss together in pairs, larger groups, and the whole class to achieve consensus. The effects of P4C on quantity and quality of interactive dialog on 180 10-year-old children in primary (elementary) mainstream classes in Scotland were investigated. Video recordings of teacher-led and pupil-pupil classroom dialog before, and seven months into, participation in the program were analyzed. Changes in intervention classes included: increased use of open-ended questions by the teacher, increased participation of pupils in classroom dialog, and improved pupil reasoning in justification of opinions. There were no changes in comparison classes. Increases were found in the amount of teacher-student dialog and the amount of student-student dialog, with decreases in the amount of teacher-whole-class talk. This study found gains in cognition, which were sustained from elementary school into high school even when no further P4C was done. Socio-emotional aspects of learning also improved. Investigation of process found changes in teacher behavior and child behavior. Implications for future research, policy and practice are outlined, particularly for enhancing consistency and expanding the program.