OBJECTIVE: To review the effects of school closures on pandemic and seasonal influenza outbreaks. DESIGN: Systematic review. DATA SOURCES: MEDLINE and EMBASE, reference lists of identified articles, hand searches of key journals and additional papers from the authors' collections. STUDY SELECTION: Studies were included if they reported on a seasonal or pandemic influenza outbreak coinciding with a planned or unplanned school closure. RESULTS: Of 2579 papers identified through MEDLINE and EMBASE, 65 were eligible for inclusion in the review along with 14 identified from other sources. Influenza incidence frequently declined after school closure. The effect was sometimes reversed when schools reopened, supporting a causal role for school closure in reducing incidence. Any benefits associated with school closure appeared to be greatest among school-aged children. However, as schools often closed late in the outbreak or other interventions were used concurrently, it was sometimes unclear how much school closure contributed to the reductions in incidence. CONCLUSIONS: School closures appear to have the potential to reduce influenza transmission, but the heterogeneity in the data available means that the optimum strategy (eg, the ideal length and timing of closure) remains unclear.