Atopic dermatitis (AD) is a multifactorial, heterogenous disease that arises as a result of the interaction between both environmental and genetic factors. Loss-of-function mutations found within the FLG gene encoding the structural protein, filaggrin, represent the most significant genetic factor predisposing to AD identified to date. In this article is reviewed the updated one of the genetic factors predisposing to AD, namely, a failure of skin barrier function undermined by mutations in the fillagrin gene. These genetic defects are responsible for enhanced permeability of the skin for allergens while epicutaneous sensitization promotes the development of atopic dermatitis.