Supernova remnants (SNR) are the most likely source of galactic cosmic rays (CRs) up to the 'knee' in the spectrum at a few PeV. The theory of diffusive shock acceleration nicely supplies a power law energy distribution with approximately the desired spectral index and with suitably high efficiency. For a SNR blast wave expanding into a typical interstellar magnetic field the predicted maximum CR energy falls short of 1 PeV, but a non-resonant plasma instability allows the CRs themselves to amplify the magnetic field by orders of magnitude to a level capable of accelerating CRs to the knee.