Exposure to allergens first occurs at body surfaces in direct contact with the environment such as the skin, airways, and gastrointestinal tract, and compelling evidence suggests that allergic inflammatory responses are profoundly influenced by the products of epithelial cells located at these sites. One such product is thymic stromal lymphopoietin (TSLP), which is capable of affecting multiple cell lineages involved in allergic reactions. In this review we discuss recent work that has provided insight into the role TSLP plays in both aberrant and protective allergic inflammatory responses, as well as regulation, associations with disease, sources, and functions of this important cytokine.