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In most theoretical massive star models, a fraction of the core undergoes convective Si burning about 10$^5$ seconds before core collapse. The exact interplay of convection and nuclear processes that comprise this phase of evolution is non-linear, so that many of the assumptions of mixing length theory and composition mixing algorithms might not be valid. Our experience with two dimensional simulations of convective O-burning (Baz\'an and Arnett 1994a; Baz\'an and Arnett 1994b) leaves us skeptical that even diffusive mixing approximations can properly address the various density, temperature, composition, and velocity inhomogeneities that should oocur in the convective flow. The structure left behind from this stage of evolution determines the progression from core collapse to core bounce and onto shock wave development and morphology. Ultimately, supernovae light curves and $\gamma$-ray lines will depend on how this stage of evolution will be treated.
