Estuaries are highly productive ecological systems that are subjected to a wide variety of human pressures. This chapter provides a comprehensive overview of estuaries structure and functioning in the context of global changes. It begins by discussing various definitions and geological origins of estuaries, emphasizing their relatively short lifespan and the challenges they face due to human activities. The diversity of estuarine systems based on geomorphology and hydrology is presented, highlighting the singularity of each system. Biogeochemical cycles, sources of organic matter, and the distribution of organisms along salinity gradients typical of estuaries are synthesized, as well as the ecosystem services they provide, including provisioning, regulating, and cultural services. Anthropogenic pressures affecting estuarine structure and functioning are briefly reviewed, namely direct habitat loss, river flow regulation and water abstraction, pollution, fisheries and aquaculture, non-indigenous invasive species, and climate change. The impacts of these pressures on biological communities and ecosystem services are discussed, with examples from various case studies. The prospective trajectories of estuaries under global change are assessed.