Although glucocorticosteroids are still the first line of treatment for chronic asthma, over the last two decades great advances have been made in understanding the pathogenesis of asthma that enabled the identification of new therapeutic targets for asthma treatment. The interleukin (IL) 5: eosinophil axis is a hallmark pathway of allergic inflammation that has received much attention. Indeed, IL-5 is known to regulate eosinophil differentiation, proliferation, priming and activation. Therefore, therapeutic agents targeting IL-5 have been generated. In this review we will discuss the effects of IL-5 on eosinophils and outline the signaling mechanism involved in IL-5-mediated effects. Furthermore, recent results from clinical trials targeting IL-5 in asthma and hypereosinophilic syndrome will be discussed and an overview of newly developed patents aimed to target IL-5 will be reviewed.