The critical nuclear physics needed to calculate the evolution of and nucleosynthesis in massive stars is reviewed with emphasis on the uncertainties. Some reaction rates - the most important and uncertain still being 12C(α,γ)16O - are important for energy generation and stellar structure. Others, like 22Ne(α,n)25Mg and individual cross sections for neutron capture determine the overall yield and distribution of the s-process for elements lighter than A = 90. Still others affect, the synthesis of individual nuclei, e.g., by the ν-process, the rp-process, and the γ-process. Weak interaction rates are important in determining the stucture of both the pre-collapse and the post-collapse supernova core. Finally, both the site for, and the nuclear physics of the r-process remains very uncertain, quite possibly because of the neglect of magnetic fields.