Salt marsh ecosystems on Cape Cod, MA have exhibited substantial changes within the last 60 years. Analyses of aerial photographs dating back to 1947 reveals that extensive marsh area loss and alterations in tidal creek structure have occurred where vegetation along the edges of tidal creeks and mosquito ditches in the low marsh has declined or disappeared. Where edge vegetation has not been lost, and where major changes in tidal inlet size have not resulted in flows that cause erosion and bank slumping, marsh area and creek structure has remained very stable. The extent of high-marsh vegetation in virtually all systems has diminished greatly, particularly since the 1980s, with the seaward edge of this zone rapidly retreating in a landward direction. In several systems, this has resulted in high marsh being replaced by barren mudflat. In others, low-marsh advancement has been able to keep pace with high-marsh retreat. These processes are discussed within the context of various biotic and abiotic factors that are the likely agents of change.