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We describe the numerical simulation of fueling and particle transport in both present and future tokamak plasmas. Models for pellet ablation and plasma density behavior after pellet injection are compared with experimental results in ISX and PDX plasmas and then extended to fusion reactor conditions. The role of fast ion ablation due to intense neutral beam injection and fusion alphas is examined along with pellet size and velocity considerations. In plasmas with high pumping efficiency (which may be obtained with divertor operation), pellet injection can significantly reduce fueling rates while maintaining more flexibility in control of the density profile than afforded by gas puffing. When fueling is dominated by gas puffing or high recycle from the walls or limiter, control of the fueling and density profiles is reduced and particle fluxes to the wall increase.