There is a longstanding recognition that asthma is a variable disease and attempts to elucidate the causes of asthma have been hampered by its genetic and phenotypic heterogeneity. The recognition that asthma constitutes several distinct phenotypes, the development of novel non-invasive biomarkers of airway inflammation and the desire to maximise information from large-scale genotyping studies have prompted new approaches to defining asthma phenotypes in adults and children. These have included statistical modelling of longitudinal symptom data in epidemiological studies and a re-examination of combinations of clinical, physiological and pathological markers that signify discrete disease entities. It is hoped that better understanding of asthma phenotypes will provide useful new insights into asthma aetiology but will also be of immense benefit in developing and tailoring asthma therapy to individual patients. This review aims to summarise traditional approaches to categorising asthma into different types and to consider novel and emerging approaches to this problem and their likely impact on understanding the causes and natural history of asthma.