Changes in the rate and direction of sand transport in space and time give rise to patterns of erosion and deposition on dunes on a range of timescales that range from individual transport events, through seasonal changes in wind direction, to decadal and longer periods of dune growth, migration, and extension. Flow acceleration results in erosion of windward or stoss slopes. Flow separation in the lee gives rise to avalanching and deposition of sand, leading to dune migration. These processes are manifested in different ways on dunes of differing types, indicating a series of interactions between dune topography and processes.