Background: Omalizumab is approved for treating severe allergic asthma from age 6, but the definition of severe asthma including a systematic assessment to rule out difficult-to-treat asthma has changed since the drug was approved in 2003. Methods: We conducted a systematic review and meta-analysis of two critical (exacerbation rate, oral corticosteroid (OCS) treatment) and eight important clinical outcomes in children, adolescents and adults, and specifically searched papers for systematic assessment of severe asthma. Results: Adults: seven studies (n = 2159) ascertaining exacerbation rate showing a 37% (95% CI 21-50) reduction in favor of omalizumab, larger than the pre-specified minimal clinically important difference (MCID) of 25%. Only one open-label study (n = 82) was identified assessing the percentage of patients experiencing reduction of OCS-maintenance treatment showing a significantly greater decrease in the omalizumab group (- 45% vs. + 18.3%, p = 0.002). Children and adolescents: four studies (n = 1551) reported data on exacerbations (no meta-analysis conducted), showed overall improvements in exacerbation rate and some passed MCID. No OCS studies were identified. No included studies provided systematic assessment of severe asthma according to current guidelines. Conclusions: Omalizumab provides clinically relevant improvements in exacerbation rate among children, adolescents, and adults and in OCS-reduction among adults. New studies incorporating a guideline-approached definition of severe asthma are warranted.