This issue of Immunology and Allergy Clinics of North America, guest edited by Dr. Rohit Katial, is devoted to Severe Asthma. Articles in this issue include: Epidemiology and Pulmonary Physiology of Severe Asthma; Linkage and Genetic Associations in Severe Asthma; Asthma COPD Overlap Syndrome (ACOS); Biomarkers in Severe Asthma; Imaging in Severe Asthma; Eosinophilic Phenotype; Neutrophilic and Pauci-immune Phenotypes; Role of Sleep Apnea and Gastroesophageal Reflux Disease in Severe Asthma; Role of Small Airways in Severe Asthma; Chronic Infection and Severe Asthma; Chronic Rhinosinusitis and Aspirin Associated Respiratory Disease; Psychosocial Factors in Severe Asthma; Traditional Therapies for Severe Asthma; and Emerging Biologics for Severe Asthma.