  I report on recent numerical simulations of the simplest field theory with
cosmic string solutions, the Abelian Higgs model. We find that random networks
of string quickly converge to a scaling solution in which the network scale
length $\xi$ increases linearly with time. There are very few loops with sizes
less than $\xi$, and the strings are smooth, showing no signs of ``small scale
structure''. We claim that particle production is the dominant energy-loss
mechanism, not gravitational radiation as previously thought. For strings in
Grand Unified Models, stringent constraints can be placed from cosmic ray
observations on the string tension $\mu$: we estimate $G\mu < 10^{-9}$, three
orders of magnitude lower than the constraint from Cosmic Microwave Background
fluctuations.
