A consensus view of the ozone time-series literature can be hard to discern as it is fragmented across different averaging times and seasons. This chapter presents the results of a systematic review and meta-analysis of mortality and hospital admission studies, structured to enable comparison of coherence across age-groups and averaging times. This is the mostrecent meta-analysis covering a full range of outcomes, combining both single- and multi-city estimates and analyzing regional variations.Methods: Medline, Embase and Web of Knowledge were searched for time-series studies on ozone, indexed to May 2011. A sifting process identified eligible single-pollutant, all year effect estimates on mortality and hospital admissions for meta-analysis, avoiding duplication by location. Single and multi-city estimates were pooled, to give summary estimates by averaging time and region. Results are expressed as 1000*ln(relative risk).Results: 182 time series studies on mortality or hospital admissions provided all year, single pollutant effect estimates. Averaging times used in the studies varied by region. 8-hour O3 was positively associated with increases in all-cause mortality: 0.34 (95% CI: 0.12, 0.56), with analogous results for 1-hour (but confidence intervals spanned zero), and 24-hour measures. Associations with cardiovascular mortality were positive, with confidence intervals above zero, for all three averaging times and supported by positive results for cardiac or stroke mortality. The associations with respiratory mortality were positive across averaging times and supported by results for mortality from specific respiratory diseases. Evidence of small study bias was found in a few of the mortality analyses, but did not affect the qualitative conclusions. 8-hour O3 was positively associated with increases in cardiovascular admissions of 0.11 ( CI -0.06, 0.27).Results were not consistent by age group or supported by clear results with more specific cardiovascular disease groupings. 8-hour O3 was positively associated with increases in respiratory admissions, 0.75 (95% CI 0.30, 1.20) with analogous results for 1-hour (but confidence intervals spanned zero), and 24-hour average ozone. Results by age-group were consistent. Summary estimates for asthma, COPD and lower respiratory infections for variousage-groups were mostly positive but the confidence intervals spanned 0. Heterogeneity and variation across WHO regions was considerable for many outcomes.Conclusions: There was reasonable evidence of associations with both respiratory and cardiovascular mortality. Evidence was stronger for associations with respiratory admissions than with cardiovascular admissions. Heterogeneity and use of different averaging times in different regions complicates interpretation of the results. Further work is needed reviewing information from multi-pollutant and seasonal models and considering potential reasons forheterogeneity.