The correct diagnosis of asthma is usually made easily and most patients respond to treatment. Approximately 5 to 10% of patients, however, have severe refractory asthma that continues to be poorly controlled despite maximal inhaled therapy. Severe asthma is not a single disease, but a collection of different phenotypes, the identification of which is crucial since this can lead to better disease management and optimal response to appropriate treatment. Additionally, specific diagnostic problems characterize asthma in the elderly and obscure the differentiation of asthma from chronic obstructive pulmonary disease (COPD). In elderly patients with longterm asthma, reversibility of airway obstruction is diminished, and a disease pattern similar to that of COPD may develop. In addition, smoking and ageing both increase bronchial hyperresponsiveness (BHR) and neutrophil numbers, resulting in asthma with a COPD phenotype. On the other hand, a subgroup of patients with COPD shows reversibility of airway obstruction associated with increased exhaled nitric oxide (NO) and sputum eosinophilia. COPD is often accompanied by BHR, and both smoking and ageing appear to be risk factors for increasing BHR, while smoking cessation improves BHR, both in patients with asthma and those with COPD. Rigid diagnostic criteria, using a combination of tests of lung function, BHR and atopy status, high resolution computed tomography (HRCT) chest scan and the newly developed biological techniques for the assessment of biomarker profiles, can facilitate the correct diagnosis and the distinction between the severe asthma phenotypes.