Coastal ecosystems such as mangroves, salt marshes, seagrass meadows, and dune vegetation play a pivotal role in mitigating the impacts of waves, currents, and storm surges through a range of interconnected biophysical mechanisms. This chapter explores the scientific principles underpinning the ability of vegetated coastal systems to dissipate hydrodynamic energy, reduce erosion, and promote sediment deposition. It examines how vegetation structure—root density, stem flexibility, canopy complexity, and spatial arrangement—influences water flow resistance and sediment trapping efficiency. Emphasis is placed on quantitative models and empirical studies that examine the interactions between vegetation and physical processes, including wave attenuation, turbulence reduction, and shoreline stabilization. The chapter further discusses the influence of geomorphological and hydrological conditions, including substrate type, tidal regime, and coastal slope, on the efficiency of bioshields. Comparative analyses between natural and restored bioshields illustrate the dynamic feedback between ecological succession and physical stability. Ultimately, understanding these mechanisms is fundamental to optimizing the design, placement, and long-term sustainability of bioshields as nature-based solutions for coastal defense. By integrating ecological and engineering perspectives, this chapter establishes a scientific framework for the functional assessment of bioshields in diverse coastal settings.