The invention provides antibodies that bind specifically to human indoleamine 2,3-dioxygenase 1 (IDO1), and methods of using the same. The antibodies are capable of binding to a sequence comprising SEQ ID NO: 1 and specifically bind to human IDO1 in formalin fixed paraffin embedded tissues. The antibodies are useful in a number of different analytical techniques, including immunohistochemistry (IHC) and immunocytochemistry (ICC).