||Coastal landscapes are often dominated by extensive tidal flats. Tidal flats are characterized by near-horizontal topography and are typically depositional environments that store sediments transported by rivers and nearshore currents. These environments support a diverse biota that modifies the erosive characteristics of the substrate and mediates sediment transport processes. Biofilms are a ubiquitous feature of intertidal mudflats and stabilize the bottom by secretion of extracellular polymeric substances and formation of a tough layer protecting the underlying sediments. Seagrasses on subtidal flats also play an important role in regulating near-bed flow and particle dynamics. A positive feedback between seagrasses, substrate stabilization, and turbidity of the water column ultimately controls the morphological stability of these landforms. In this chapter, an overview of the sedimentological and physical processes acting on tidal flat sediments have been presented. The effects of biofilms and seagrasses on tidal flat substrates and sediment transport processes are then introduced.