The invention relates to antibody molecules and antigen-binding portions thereof which bind specifically to glucocorticoid-induced TNF receptor (GITR). In particular aspects of the invention, the antibody molecules specifically bind to human GITR and cynomolgus monkey GITR. The anti-GITR antibody molecules of the invention have been developed and optimized using CDR sequences derived from a murine anti-GITR antibody 6C8. Medical uses of the antibody molecules are disclosed.