Large-eddy simulations (LES) were performed on the gas flow in a Stairmand high-efficiency cyclone at Re = 280,000. The LES realistically represent the 3-D, time-dependent flow, including phenomena such as vortex core precession, and vortex breakdown. Quantitatively good agreement with measured velocity profiles is observed, for both the time-averaged velocities and the velocity fluctuation levels. The single-phase LES formed the starting point for modeling solid particle motion in the cyclone based on one-way coupling between the gas flow and the particles. It is shown that some details of the flow in the relatively small region in the vicinity of the inlet have strong influence on the separation process. Due to the long residence times of particles inside the cyclone, the concurrent simulation of gas flow and particle motion is a lengthy computational process. Therefore, three alternative ways of modeling particle motion were explored: a frozen-field approach, an eddy-lifetime model, and a periodic-flow approach. The results of these approaches are compared to the results of the concurrent simulation.