River deltas are subaqueous and subaerial coastal accumulations of river-derived sediments adjacent to, or close to, the source river. The word “delta,” however, is used in a more general sense to describe any feature resulting from this type of marginal accumulation, including in lakes, lagoons, ponds, mining tails, and reservoirs. Most river deltas are formed on the margins of marine basins. River deltas vary considerably in size, and some are the largest coastal landforms in the world. In addition to fluvial sediments, delta deposits sometimes include marine or along-shore derived sediments transported by waves and currents. Deltas form where the hydrodynamic conditions in the receiving basin are not energetic enough to disperse all or the bulk of the sediment brought in by rivers. Sediment transported through deltas contributes to deposition on adjacent coasts, continental shelves, and marine basins. Much of the early research on modern deltas focused on their oil- and gas-bearing potential and how they are analogs for ancient deltas in the rock record. There has been a shift, however, toward increasingly more diverse and cross-disciplinary research on deltas. Deltas are complex landforms. Recent research has shown that deltas also act as filters, sinks, and reactors for continental materials, including carbon, in transit to the ocean. Deltas are home to nearly six hundred million people. They commonly have highly productive soils, rich and biodiverse ecosystems, and offer a wide range of ecosystem services such as coastal defense, drinking water supply, recreation, green tourism, and nature conservation. Many deltas support intense agriculture and fisheries and are food baskets for many nations. Industry and transport in some deltas are also very important, leading to the development of major urban centers, ports, and harbors. Deltas are characterized by low topography and thus particularly vulnerable to catastrophic river floods, tsunami, cyclones, subsidence, and global sea-level rise. This vulnerability is increasing as a result of reduced sediment flux from rivers and various other modifications caused by human interventions. Although deltas may develop resilience and adapt to changes in sediment supply and sea level, commonly by reorganizing their channels and their patterns of sedimentation, human impacts coupled with the effects of climate change are rendering many deltas economic and environmental hotspots. A better understanding of delta dynamics and vulnerability, and a lot of political goodwill, are needed to implement adaptive delta management, and delta restoration and rehabilitation strategies.