A level set formulation is used to simulate profile evolution in etching and deposition processes. This approach, an alternative to string algorithms to track a moving front, is coupled to a model that computes etch rates based on first principles. Results are presented for silicon etching in a chlorine plasma. Etch profiles for various ratios of neutral to ion flux, ion anisotropy parameters, trench widths, and mask thickness are presented. Simulations capture the reactive ion etching lag effect and shadowing effects due to mask thickness.