This paper introduces the notion of formation input-to-state stability in order to characterize the internal stability of leader-follower formations, with respect to inputs received by the formation leader. Formation ISS is a weaker form of stability than string stability since it does not require inter-agent communication. It relates group input to internal state of the group through the formation graph adjacency matrix. In this framework, different formation structures can be analyzed and compared in terms of their stability properties and their robustness.