The asthma phenotype can be described using a combination of the following: symptom type, pattern and severity; markers of atopy; and measurement of bronchial responsiveness. Because of the very nature of the disease, symptoms of asthma are variable in both the short- and the long-term, and the natural history of the disease is such that symptoms in an individual may evolve over time through different patterns. Although atopy appears to be a life-long attribute resulting from an early life switching to a TH2 immune response, the surrogate markers of atopy each are subject to their own time-related determinants and patterns of change with age. Bronchial responsiveness in childhood is neither specific nor sensitive for asthma, and although showing good short-term repeatability, can vary widely when measured over a period of months or years. Stimuli for responsiveness testing should be chosen which can be inhaled safely in high doses so as to allow an end point to be reached by as many subjects within a population as possible, and individuals may have to be tested repeatedly over time so as to avoid misclassification.