We consider the problem of routing traffic to optimize the performance 
of a congested network.  We are given a network, a rate of traffic 
between each pair of nodes, and a latency function for each edge 
specifying the time needed to traverse the edge given its congestion; 
the objective is to route traffic such that the sum of all travel 
times---the total latency---is minimized.

In many settings, it may be expensive or impossible to regulate 
network traffic so as to implement an optimal assignment of routes.  
In the absence of regulation by some central authority, we assume that
each network user routes its traffic on the minimum-latency path 
available to it, given the network congestion caused by the other 
users. In general such a ``selfishly motivated'' assignment of traffic 
to paths will not minimize the total latency; hence, this lack of 
regulation carries the cost of decreased network performance.

In this paper we quantify the degradation in network performance due 
to unregulated traffic.  We prove that if the latency of each edge is 
a linear function of its congestion, then the total latency of the 
routes chosen by selfish network users is at most $\frac{4}{3}$ times 
the minimum possible total latency (subject to the condition that all 
traffic must be routed).  We also consider the more general setting 
in which edge latency functions are assumed only to be continuous and 
nondecreasing in the edge congestion.  Here, the total latency of the 
routes chosen by unregulated selfish network users may be arbitrarily 
larger than the minimum possible total latency; however, we prove 
that it is no more than the total latency incurred by optimally 
routing {\em twice} as much traffic.