Tropical rivers provide about 60% of the global transport of organic and inorganic carbon from continents to the coastal zone. These inputs combine with organic material from productive mangrove forests, seagrass beds, and coral reefs to make tropical coastal ecosystems important components in the global carbon cycle. Carbon exchange has been measured over multiple spatial scales, ranging from the transport and fate of terrestrial organic matter to the coastal zone, export of organic matter to the open ocean, exchange of leaf litter between mangroves and adjacent seagrass beds, to movement of carbon (at a scale of meters) between adjacent saltmarsh and mangrove habitats. Carbon is exchanged directly as particulate or dissolved material, or through migration of animals or through a series of predator-prey interactions known as trophic relay. This chapter first examines riverine carbon inputs to the tropical coastal zone, and how this material is processed in estuaries. The mechanisms and extent of carbon exchange among tropical coastal ecosystems are then discussed, showing their importance in ecosystem carbon budgets, and the implications for faunal and microbial communities.