It is recognised that airway inflammation is key to asthma pathogenesis. Biopharmaceutical approaches have identified new therapies that target key cells and mediators that drive the inflammatory responses in the asthmatic lung. Such an approach resulted in the development of biologics targeted at inhibition of interleukin (IL)-4, IL-5 and IL-13. However, early clinical trials with these biologics in patients with asthma were, for the most part, disappointing even though they were highly effective in animal models of asthma. It is becoming apparent that significant clinical effects with anti-cytokine-based therapies are more likely in carefully selected patient populations that take asthma phenotypes into account. The development of discriminatory biomarkers and genetic profiling may aid identification of such patients with asthma. This review summarises the current evidence, demonstrating the effectiveness or otherwise of the targeting of IL-5 in patients with asthma.