Asthma is a syndrome characterized by airway inflammation, airway hyperresponsiveness (AHR) and airway obstruction. The diagnosis asthma comprises a very heterogeneous patient population. In recent years different strategies have been developed to classify the patients into different phenotypes. The phenotypes can be based on clinical findings, etiology and triggers or be defined by the type of airway inflammation. These classifications are still very crude and confusing and often a clear-cut differentiation between different described phenotypes is not possible. Still some phenotypes depend on different pathophysiological mechanisms and can differ profoundly in their clinical course. Also for some of these phenotypes specifical treatpeuthic options are available. Therefore a good definition and classification of asthma phenotypes is important in regard of diagnosis, prognosis and therapy of these patients.