Severe asthma is a form of asthma that responds poorly to currently available medication, and its patients represent those with greatest unmet needs. In the last 10 years, substantial progress has been made in terms of understanding some of the mechanisms that drive severe asthma; there have also been concomitant advances in the recognition of specific molecular phenotypes. This ERS Monograph covers all aspects of severe asthma – epidemiology, diagnosis, mechanisms, treatment and management – but has a particular focus on recent understanding of mechanistic heterogeneity based on an analytic approach using various ‘omics platforms applied to clinically well-defined asthma cohorts. How these advances have led to improved management targets is also emphasised. This book brings together the clinical and scientific expertise of those from around the world who are collaborating to solve the problem of severe asthma.