Thermally activated grain boundary migration (GB) is important in grain growth and thermo-mechanical processing of materials, but very little is known about the atomic-scale mechanisms involved. The purpose of this paper is to identify atomic-scale GB migration mechanism and investigate their dependence on GB structure. High-angle tilt and general GBs are investigated at elevated temperatures by high-resolution transmission electron microscopy (HREM) in Au and Al bicrystalline thin films. Digital analysis of HREM video recordings is used to detect atomic-scale structural changes at migrating GBs. GB motion typically is not smooth, but involves sharply varying speeds and thermally activated spatial fluctuations. Collective effects in GB migration are shown to exist and several different migration mechanisms are identified. Atomic-scale GB migration is found to depend on the macroscopic GB geometry as well as details of the interatomic interactions.