Herodotus is our main source for the Persian Wars (499-479 B.C.). Behind the scenes in these wars, policy had to be determined and put into practice, not only between poleis within Greece itself but also with the Persian Empire. Naturally, interstate relations between Greece and Persia were present before the onset of the war as well as during it and constitute an essential dimension of the conflict. The workings of such diplomatic conduct are far from self-explanatory, however. In order to investigate them thoroughly and reveal the hinges that were crucial in determining policy, the interstate relations between Athens and the Persian Empire are here compared to those between Sparta and the Persian Empire, before and during the Persian Wars, as described in Herodotus’ Histories. By looking at the methods and aims of diplomacy, the sending of envoys, and the role of deserters, factors like location, urgency of immediate threat, pragmatism, ideology and realism are uncovered as playing a prominent role.