The rapid and precise direct-write growth of nanoscale features by electron-beam-induced deposition (EBID) and etching (EBIE) requires the optimization of the growth parameters to maintain nanoscale feature dimensions. The tremendous and complex EBID/EBIE parameter space includes the precursor gas pressure, primary electron beam energy, electron beam current, surface diffusion rates of adsorbed precursor species, thermal effects on desorption, and the cascade of electron species produced by elastic and inelastic scattering processes. These variables affect the probability of precursor dissociation and hence determine the feature growth velocity and the size of the structure through a series of complex, coupled nonlinear interactions. In this presentation, a variety of experimental studies will be presented to demonstrate the various electron-gas, gas-solid, and electron-solid interactions that are germane to the electron beam induced processing technique. A dynamic computer simulation based on Monte-Carlo calculation sequences will then be described and compared to various experimental observations. Finally, several nanoscale device applications will be demonstrated.