Nanocrystalline metals, i.e., metals in which the grain size is in the nanometer range, have a range of technologically interesting properties including increased hardness and yield strength. We present atomic-scale simulations of the plastic behavior of nanocrystalline copper. The simulations show that the main deformation mode is sliding in the grain boundaries through a large number of uncorrelated events, where a few atoms (or a few tens of atoms) slide with respect to each other. Little dislocation activity is seen in the grain interiors. The localization of the deformation to the grain boundaries leads to a hardening as the grain size is increased (reverse Hall-Fetch effect), implying a maximum in hardness for a grain size above the ones studied here. We investigate the effects of varying temperature, strain rate, and porosity, and discuss the relation to recent experiments. At increasing temperatures the material becomes softer in both the plastic and elastic regime. Porosity in the samples result in a softening of the material; this may be a significant effect in many experiments. [S0163-1829(99)05941-X].