A formalism is presented for investigating transition pathways and transition probabilities for rare events in complex systems. Specifically, we show how driven simulations may be used to calculate relative reaction rates between stable states by means of nonequilibrium work measurements. Our relation between equilibrium and nonequilibrium rates may further be optimized by means of bidirectional estimators. A simple proline system is used to numerically illustrate the results. (C) 2011 Elsevier B.V. All rights reserved.