The present invention provides binding molecules that specifically bind to GITR, e.g., human GITR (hGITR), on T cells and dendritic cells. Binding molecules of the invention are characterized by binding to hGITR with high affinity, in the presence of a stimulating agent, e.g., CD3, are agonistic, and abrogate the suppression of Teff cells by Treg cells. Various aspects of the invention relate to binding molecules, and pharmaceutical compositions thereof, as well as nucleic acids, recombinant expression vectors and host cells for making such binding molecules. Methods of using a binding molecule of the invention to detect human GITR or to modulate human GITR activity, either in vitro or in vivo, are also encompassed by the invention.