The present invention relates to immunoglobulin single variable domains that bind GITR and more in particular to polypeptides that comprise or essentially consist of one or more such immunoglobulin single variable domains; to nucleic acids encoding such polypeptides; to methods for preparing such polypeptides; to compositions and in particular to pharmaceutical compositions that comprise such polypeptides for prophylactic therapeutic or diagnostic purposes. In particular the polypeptides of the present invention enhance the biological activity of GITR.