Present status of diffusive shock acceleration

Diffusive shock acceleration (DSA) is now widely accepted as the model to explain the production of cosmic rays (CRs) in a wide range of astrophysical environments. Despite initial successes of the theory in explaining the energetics and the spectrum of CRs accelerated by supernova remnants, there still remain some unresolved issues such as particle injection out of the thermal plasma at shocks, CR diffusion due to the self-generated MHD waves and yet-to-be-detected gamma-ray emission due to the ionic CRs. Recent technical advancements to resolve these issues are reviewed.