Cysteinyl leukotrienes (cysLTs) are a class of closely structurally related lipid molecules, originally described as slow-reacting substance of anaphylaxis, with a myriad of biologic functions. These activities include producing smooth muscle contraction and mucus secretion, recruiting allergic inflammatory cells, modulating cytokine production, influencing neural transmission, and altering structural changes in the airway. Administration of cysLTs to animals and human subjects reproduces many features of allergic inflammation and asthma. Leukotriene (LT) blockers have independent efficacy in asthma and improve pulmonary function when added to inhaled steroids. Conversely, blockade of this pathway both in animals and in human subjects results in important reductions in inflammation and its consequences and might reduce structural changes of remodeling. These data collectively make a compelling case for an important role of cysLTs in airway inflammation and asthma. However, the magnitude of effect of anti-LTs is smaller than that of corticosteroids, and there is more variability in benefit of LT blockade than is seen with inhaled steroids. In addition, adding anti-LTs to inhaled steroids in asthmatic patients does not appear to produce added anti-inflammatory benefit. Genetic polymorphisms and environmental factors, such as tobacco smoke exposure, might underlie some of the heterogeneity of response to LT blockers.