BACKGROUND Epithelial cell-derived mediators have emerged as key players for instigating local remodeling and the associated cellular inflammation in asthmatic airways. In particular, the epithelial-derived cytokine, thymic stromal lymphopoietin (TSLP), has been identified as a master switch for allergic inflammation. METHODS TSLP is expressed by structural and immune cells at the site of allergen entry in the airways. Stimuli for release of TSLP include common triggers of asthma symptoms, and TSLP levels correlate with disease severity. TSLP regulates helper T cell 2 (Th2) humoral immunity through upregulating OX40L on dendritic cells (DCs), which drives Th2 lymphocytes; however, activation of several other cells by TSLP also supports the development of Th2 inflammation. Animal models of asthma demonstrate that increased levels of TSLP can induce many of the characteristics of asthma. CONCLUSIONS The work conducted to date supports a critical role of TSLP in the pathogenesis of allergic asthma. The first clinical trial to block the downstream effects of OX40L has shown reduced levels of circulating IgE and airway eosinophils, confirming the importance of TSLP-induced OX40L levels on DCs. Clinical trials with TSLP blockade are underway and will unequivocally confirm whether TSLP is indeed a key driver of allergic inflammation in asthma.