Biomarkers of eosinophilic inflammation have become increasingly clinically relevant with the advent of targeted biological treatments for severe asthma, but also for guiding the choice of inhaled steroids in the management of COPD. Measuring airway eosinophils in induced sputum is the gold standard for detecting and quantifying eosinophilic airway inflammation, but requires a specific set-up for immediate processing, thus is only available in highly specialised clinics. The blood eosinophil level may be used as a surrogate marker, as well as F_ENO. Eosinophilic biomarkers may be used for different purposes in the clinical setting: inflammatory phenotyping, predicting future risk of exacerbations, predicting response to treatment, assessing the response to treatment, assessing adherence and adjusting treatment. With the advent of biologics, an increased focus on biomarkers reflecting activation of specific pathways can be expected, such as F_ENO as a marker of IL-13 activation. Finally, other biomarkers of eosinophilic inflammation may become more relevant in the future, for assessing eosinophilic activation status.