Coastal ecosystems are highly valued as key economic and cultural assets for society. They provide a wealth of ecosystem functions including storm protection, nutrient cycling, carbon sequestration, water filtration, detrital processing, fisheries, food web support, biodiversity and wildlife habitat. Rapidly growing populations and expanding development are intensifying pressures on these valuable ecosystems. Sea-level rise and other predicted effects of climate change are expected to exert even greater pressures on these ecosystems at the edge of land and sea, exacerbating erosion, degrading habitat, and accelerating shoreline retreat. Historically, society’s responses to threats from erosion and shoreline retreat to coastal development and infrastructure have relied heavily on armoring and other engineered coastal defenses. Coastal armoring structures are deployed on all types of open and sheltered coasts in a wide range of tidal and wave conditions, as well as in onshore and offshore locations. The majority, especially those constructed for protection against erosion, are constructed upon coastal landforms dominated by soft sediments, including beaches, dunes, friable coastal bluffs, estuarine and tidal creek channels adjacent to marshes, mudflats, harbors, and inlets. Armoring fixes shoreline position, constraining possible responses and evolution of soft sediment ecosystems to changes in sea level and other dynamic coastal processes. As an alternative to armoring, living shorelines, involving strategic placement of structural or organic material (e.g., rock, oyster reef, grass, dunes) are used in some areas to reduce erosion and enhance wetland habitat. Managed retreat of infrastructure is also used along selected shorelines. Despite widespread use of armoring on all types of shorelines, the lack of information about the ecological effects of shoreline armoring and of alternatives to armoring, such as living shorelines and managed retreat, represents a critical gap in the knowledge needed to sustain coastal ecosystems and the functions that they provide.