Pycnonuclear reactions occur in high density matter when nuclei are frozen in lattice structures. Such matter characterizes the cores of white dwarfs, and the crusts of accreting neutron stars. While pycnonuclear reactions in white dwarf matter are dominated by 12C and 16O induced fusion processes, pycnonuclear reactions in neutron star crust matter are dominated by fusion between very neutron rich carbon, oxygen, and neon isotopes at the limits of stability. In a post X-ray burst neutron star binary system ashes from the burst are forced deep into the crust by the weight of freshly accreted matter. With increasing density the remnant abundance distribution is changed by a host of electron capture reactions and neutron emissions, leading to the effective processing of the ashes into very neutron rich nuclei in the carbon to magnesium range. Near nuclear matter densities the nuclei are forced into a lattice, surrounded by a neutralizing degenerate electron gas. Though held in a solid lattice, reactions are still possible due to the reduction of Coulomb potential by screening and the overlap of nuclear wave functions between lattice sites. This type of reaction is highly density dependant, and unlike the more familiar thermonuclear counterpart, is not strictly temperature dependent