Regulatory T cells (Treg) play a crucial role in the regulatory mechanisms of immune system. They are responsible for the induction and maintenance of immune tolerance. They are also involved in the pathogenesis of autoimmune and allergic diseases and implicated in transplant rejection and immunopathology of cancers. Treg cells constitute the population of lymphocytes heterogeneous in their phenotype. CD4+CD25+Foxp3+ cells are most important among Treg subsets. Modification of Treg actvity may be useful in the therapy of different diseases, including allergic disorders.