  We present an algorithm for finding the probabilities of rare events in
nonequilibrium processes. The algorithm consists of evolving the system with a
modified dynamics for which the required event occurs more frequently. By
keeping track of the relative weight of phase-space trajectories generated by
the modified and the original dynamics one can obtain the required
probabilities. The algorithm is tested on two model systems of steady-state
particle and heat transport where we find a huge improvement from direct
simulation methods.
