Eosinophils are increasingly being recognised as an important characteristic feature of COPD. Patients with COPD and eosinophilic inflammation tend to respond to steroid therapy; however, many questions remain regarding the optimum measurement. Eosinophilic inflammation may be defined based on various sampling techniques, including eosinophil levels in blood, sputum, bronchoalveolar lavage or biopsy, which leads to inconsistencies in its definition. Blood eosinophils may increase in conjunction with sputum eosinophils during COPD exacerbations and therefore may be a good surrogate marker of airway eosinophilic inflammation. However, the timing of the blood eosinophil measurement, the stability of the eosinophil count and the threshold used in different studies are variable. The use of blood eosinophil count to direct biological therapies in COPD has also had variable outcomes. Eosinophilic inflammation has an important role in COPD management; however, its use as the optimum biomarker still needs further investigation.Key pointsEosinophilia may play a significant role in the pathogenesis of COPD.Eosinophilic inflammation in COPD can be steroid responsive; however, eosinophilic inflammation is variable, and caution needs to be taken with measurements and the thresholds used.The long-term effects of reducing eosinophil levels in COPD is unclear.Educational aimsTo explore current knowledge of eosinophils in COPD.To explore the relationship between eosinophilia and corticosteroid use.To understand the limitations of assessing and using eosinophilia in COPD.