During the past decade exacerbations of asthma have increasingly been recognized a as primary outcome parameter in clinical research. However, comparison of results is hampered by the use of different definitions for asthma exacerbations in published reports. The purpose of this review is to describe the consequences of using different definitions and to propose possible solutions.The use of different definitions of asthma exacerbations not only influences incidence rates, but also affects patient identification and risk assessment. The American Thoracic Society/European Respiratory Society and the World Health Organization independently proposed consensus definitions based on clinical symptoms and management. This needs to be complemented by a prospective definition that may support clinicians in their treatment decisions.Currently there is no commonly accepted definition for asthma exacerbations. A prospective definition is desirable. This could be obtained by phenotyping asthma exacerbations based on clinical signs, lung function parameters and possibly other biomarkers.