We investigate the impact of Stackelberg routing to reduce the price of anarchy in network routing games. In this setting, an fraction of the entire demand is first routed centrally according to a predefined Stackelberg strategy and the remaining demand is then routed selfishly by (nonatomic) players. Although several advances have been made recently in proving that Stackelberg routing can, in fact, significantly reduce the price of anarchy for certain network topologies, the central question of whether this holds true in general is still open. We answer this question negatively by constructing a family of singlecommodity instances such that every Stackelberg strategy induces a price of anarchy that grows linearly with the size of the network. Moreover, we prove upper bounds on the price of anarchy of the largest-latency-first (LLF) strategy that only depend on the size of the network. Besides other implications, this rules out the possibility to construct constant-size networks to prove an unbounded price of anarchy. In light of this negative result, we consider bicriteria bounds. We develop an efficiently computable Stackelberg strategy that induces a flow whose cost is at most the cost of an optimal flow with respect to demands scaled by a factor of 1 + √ 1 − . Finally, we analyze the effectiveness of an easy-to-implement Stackelberg strategy, called SCALE. We prove bounds for a general class of latency functions that includes polynomial latency functions as a special case. Our analysis is based on an approach that is simple yet powerful enough to obtain (almost) tight bounds for SCALE in general networks. 1. Introduction. Over the past years, the impact of the behavior of selfish, uncoordinated users in congested networks has been investigated intensively in the theoretical computer science and operations research literature. In this context, network routing games have proved to be an appropriate means of modeling selfish behavior in networks. The basic idea is to model the interaction between the selfish network users as a noncooperative game. We are given a directed graph with latency functions on the arcs and a set of origin-destination pairs, called commodities. Every commodity has a demand associated with it that specifies the amount of flow that needs to be sent from the respective origin to the destination. We assume that every demand represents a large population of players, each controlling an infinitesimal small amount of flow of the entire demand (such players are also called nonatomic). The latency that a player experiences to traverse an arc is given by a (nondecreasing) function of the total flow on that arc. We assume that every player acts selfishly and routes his flow along a minimum-latency path from its origin to the destination; this corresponds to a common solution concept for noncooperative games, that of a Nash equilibrium (here Nash or Wardrop flow; see Wardrop [37]). In a Nash flow, no player can improve his own latency by unilaterally switching to another path.It is well-known that ...