Abstract The hydrodynamic shock origin of cosmic rays in the envelope of a Type I presupernova star is reviewed. The spectrum produced by the relativistic hydrodynamic shock is one power of E steeper than observed and so is unlikely to be the primary source of cosmic rays. On the other hand the possibility of accelerating ultrahigh energy particles to ≳ 10 18 eV is unique to the shock mechanism and currently no other suggested galactic or extragalactic site is likely. The nonrelativistic hydrodynamic supernova explosion shock becomes relativistic at an external mass fraction of (1-F) = 3 × 10 −6 of the star that is composed primarily of helium plus heavier nuclei. The resulting ejected relativistic energy, (1-F) M Θ c 2 ≅ 6 × 10 48 ergs per SNI is roughly 1 5 that necessary to explain the Galactic cosmic ray energy. The resulting spectrum becomes, N(>E) ∝ (1-F) ∝ E −2.5, steeper than E −1.6 observed. The heavy nuclei are partially spalled in the shock transition and partially resynthesized in the postshock expansion for E ≲ 10 15 eV dependent upon the large number of pairs in the post-shock fluid. Above this energy the shock progresses in the magnetized photosphere. The high energy limit is ≅ 10 21 eV due to the coronal density of the presupernova star. The objection to SN shock accelerated cosmic rays by adiabatic deceleration is questioned on the basis of the Alfvén wave scattering conditions. Ultrahigh energy particles escape because the wave excitation energy density is too low in the dimension of many Larmor radii necessary for scattering back to the SN remnant. Others escape if the energy density is too high. For all others between these two limits the immediately following matter of lower velocity and greater mass compresses and energizes previously trapped highe renergy particles, allowing them to escape at energies still higher than originally shock ejected from the supernova. The so-called piston that drives the envelope shock is the same, i.e. the SN bulk ejected matter, or total kinetic energy of the ejecta of (≅ 2 × 10 51 ergs) as drives the ISM plasma shock. The efficiency for the envelope shock is (CR energy)/(bulk energy) ≅ 1 300 . For the Alfvén wave ISM shock to have this same efficiency requires that the spectrum of nonrelativistic particles, E > 100 keV, v shock ≅ 3 × 10 8 cm s −1, is flatter than N(>E) ∝ E −1.62, which is predicted. If there are greater loses in the ISM from the plasma shock such as electron and ion heating and bulk kinetic energy or a steeper nonrelativistic spectrum, the hydrodynamic envelope ejecta would be larger.