We consider effects of pre-existing, large-scale turbulence, 
     upstream of a shock, on the magnetic field and 
     the acceleration of charged particles.   
     Turbulent density fluctuations upstream of 
     the shock have a large effect on the magnetic field downstream 
     (Giacalone and Jokipii, Ap. J., 633, L41, 2007).   For high 
     Alfven Mach number shocks, the downstream magnetic field is 
     amplified considerably above the value obtained from the shock 
     jump conditions.  These effects may provide a robust and natural 
     understanding of recent observations at supernova shocks.  
     
     The magnetic-field amplification implied by our simulations 
     should exceed factors of 100, consistent with observed 
     X-rays from supernova remnants, which require magnetic fields 
     of 100 mu G.  These are much larger than expected from the shock 
     jump conditions. The upstream field is not amplified, 
     so cosmic-rays with energies approaching the 
     ``knee'' in the spectrum require rapid acceleration, which can occur  
     at the quasi-perpendicular part of the supernova blast wave, 
     where the turbulent field-line mixing plays a large role.  
     
     We have carried out a global test-particle simulation of 
     acceleration at a spherical blast wave propagating into a 
     uniform magnetic field.   We find that although most of 
     rapid particle acceleration occurs in the "equatorial" band, 
     where the upstream magnetic field is quasi-perpendicular, 
     the ongoing temporal evolution of the shock brings most of the 
     particles to the quasi-parallel "polar" part of the shock.   
     This is in agreement with the observational constraints reported  
     by Rothenflug, et al (A & A, 4225, 121, 2004), and allows the rapid 
     acceleration at the quasiperpendicular shock.  
     
     We conclude that a model in which the magnetic-field amplification 
     occurs because of the upstream turbulence and rapid acceleration 
     to the knee occurs at the quasi-perpendicular part of the shock 
     is consistent with the observations.  Amplification of the 
     upstream magnetic field is not necessary in this model.