Salt marshes are dynamic, productive ecosystems that reside at the margin of land and sea. Although expanses of these valuable systems have been lost to human development, many marshes have been stable despite rising sea levels. This apparent resilience results from nonlinear feedbacks that influence sediment accumulation and marsh migration into uplands to maintain marsh area. However, predictions of accelerating sea-level rise (SLR) rates lead to uncertainty about whether marshes can keep pace with SLR in the future. Furthermore, human activities, such as river damming and shoreline armoring may exacerbate the problem by limiting sediment supply and blocking upland migration. Future adaptation and mitigation strategies must be guided by a thorough understanding of how these systems respond to SLR under a given set of environmental conditions. I propose research that will contribute to this knowledge by comparatively evaluating how salt marshes have already responded to SLR. Broadly, I plan to evaluate changes in marsh cover across space and time at three research sites along the US East Coast associated with the NSF Long-Term Ecological Research (LTER) Program. I will analyze land cover data and aerial photographs for spatial changes in marsh cover at these sites, investigate whether rates of change have increased through time, and relate those changes to ecological variables collected through the LTER program. My goal is that the inferences developed through these synthetic analyses will broaden our understanding of the processes controlling loss of marshes, thereby enhancing our ability to preserve them in the future.