Sediments are an important habitat component in aquatic ecosystems, as they provide a substrate or support medium for aquatic organisms. In addition, sediments are biogeochemically active, in that they may transport organic or inorganic substances attached to their surfaces, and because they provide a large surface for abundance of microbes. Sediment erosion, transport, and accumulation affect the geomorphology of aquatic ecosystems, and sediment supply determines the lifespan of lakes. The mobilization of sediment and the quality of sediment are partly under human control, and human-mediated changes in sediment transport are the cause of extensive changes in both flowing waters and lakes. Technical advances in the measurement of sediment quality and quantity are still being made; the future offers greater insights into the movement and biogeochemistry of sediments.