Asthma causes a substantial burden of morbidity and mortality, affecting 300 million people worldwide - a figure predicted to increase to 400 million by 2025. Despite the availability of a variety of treatment options and detailed treatment guidelines, many patients with asthma, and in particular those with severe persistent asthma, remain inadequately controlled. Approximately 50-80% of severe asthma has an allergic component, with immunoglobulin E (IgE) playing a role in the underlying allergic inflammatory cascade. Omalizumab is a humanised monoclonal anti-IgE antibody that targets IgE and partially inhibits the inflammatory cascade. Clinical trials have demonstrated that omalizumab added to standard asthma therapy reduces exacerbations and emergency visits with concomitant improvements in asthma control and quality of life in patients with moderate-to-severe and severe persistent allergic (IgE-mediated) asthma. Add-on omalizumab is indicated for the treatment of patients with inadequately controlled moderate-to-severe (US label) and severe (EU label) persistent allergic asthma despite treatment with high-dose inhaled corticosteroids (and in the EU, high-dose inhaled corticosteroids plus a long-acting beta2-agonist). Within this highly-targeted patient population, analyses have been unable to identify pre-treatment clinical characteristics that are predictive of a greater response to omalizumab. In contrast, assessment of response to omalizumab following 16 weeks of treatment appears to be reliably judged by physicians in clinical trial settings and may be a feasible means of selecting patients who should continue treatment.