One of the most unexpected results from the Mars Global Surveyor mission was the discovery of strong, spatially variable remanent magnetization in the crust. The peak intensities of the crustal magnetizations on Mars are an order of magnitude greater than typical for Earth rocks at spacecraft altitude. However, the crust within the younger impact basins does not preserve any magnetization above the detection limits and may suggest that shock waves generated during impact events demagnetized large sections of the Martian crust. The timing and generation of the dynamo on Mars places fundamental constraints on the thermal evolution of the planet. Therefore, the magnetic signatures in the crust and in meteorites place important constraints on the evolution of Mars. However, interpreting the complex patterns in the Martian crust (below) requires a better understanding of the effect of shocks on magnetization.