Asthma is clearly related to airway or blood eosinophilia, and asthmatics with significant eosinophilia are at higher risk for more severe disease. Eosinophils actively contribute to innate and adaptive immune responses and inflammatory cascades through the production and release of diverse chemokines, cytokines, lipid mediators and other growth factors. Eosinophils may persist in the blood and airways despite guidelines-based treatment. This review details eosinophil effector mechanisms, surface markers, and clinical outcomes associated with eosinophilia and asthma severity. There is interest in the potential of eosinophils or their products to predict treatment response with biotherapeutics and their usefulness as biomarkers. This is important as monoclonal antibodies are targeting cytokines and eosinophils in different lung environments for treating severe asthma. Identifying disease state-specific eosinophil biomarkers would help to refine these strategies and choose likely responders to biotherapeutics.