Collisionless shocks are the most powerful particle accelerators in the universe. Generated by violent events in supernovae, extra-galactic radio jets, gamma-ray bursts, and elsewhere, they are believed responsible for the production of extremely relativistic particles, including cosmic rays – the most energetic particles ever observed. Recent broad-band photon observations of young supernova remnants, and other observations, have given credence to the idea that shocks are highly efficient and nonlinear. Observations also suggest that shocks can amplify ambient magnetic fields by factors of 100 or more! I will describe nonlinear shock acceleration and outline some of the latest efforts to understand this mechanism, including magnetic field amplification.