SUMMARY. The correct diagnosis of asthma is usually made easily and most patients with asthma respond to therapy. Approximately 5-10% of patients with asthma, however, have disease that is difficult to control despite administration of maximal doses of inhaled medications. It appears that asthma is a heterogeneous disorder which presents not as a single disease but rather as a complex of multiple, separate syndromes that overlap. Although the various different phenotypes of asthma have been long recognized, they are still poorly characterized. Improved phenotypical characterization and understanding of the underlying pathobiology are necessary for linkage of specific genotypes with clinical disease manifestations, for possible development of biomarkers and for devising advanced, phenotype-targeted asthma treatment. This review reports on the asthma phenotypes that have been best described and analyses the methods used to define them. Pneumon 2010, 23(3):260-292.