Recirculating aquaculture systems (RAS) will play an important role in the future of the seafood industry because they can be operated more independently of the environment. On the one hand, they reduce environmental impact by recycling water and nutrients. They are also more protected from the environment, making them more resilient to the challenges of climate change. Applications include hatcheries, nurseries and grow-out operations. To date, there is no single standard RAS design. There is also a wide range of filter designs for similar functions using different technologies. However, there is agreement on the basic elements required. These include (1) pumping: to circulate the water between basins and water treatment; (2) solids separation: to remove faecal matter and other particles from the water; (3) biofiltration: to convert and/or remove dissolved nutrients; (4) gas exchange: to oxygenate the water and/or remove gases; and (5) disinfection: to kill or remove pathogens from the water. In simple terms, each sink or source must be balanced to maintain water quality. As the water renewal rate is part of this equation, its level determines the technology required. Closed RAS with a renewal rate of less than 10% of the system volume per day or even less require additional filtration systems such as denitrification, dephosphorisation, backwash water recycling and ion composition control. Care must be taken when designing the hydrodynamics of the system. A distinction must be made between the velocity, hydraulic residence time and flow pattern of the water in the tank. The first has mainly to meet the biological requirements of the farmed species, but is also important for the transport of settled particles. The other two are important for the removal of suspended particles and dissolved substances such as ammonia and carbon dioxide. The design of the tank will affect the flow model to be used. For simplified design calculations, a fully mixed model can be considered as the best approximation for round tanks and a plug flow model for raceways. It is important to use the right model to design the water treatment correctly. A trend is also to consider higher recirculation rates between the basin and the water treatment to achieve better water quality and less daily fluctuations. Technology has developed rapidly in recent decades. In particular, the use of new sophisticated measurement systems at reasonable cost is providing a much broader data base that is improving the understanding of biological and chemical processes in RAS. Examples include screening for fluctuations and shifts within the microbiome or the ionic composition of water. Known problems such as hydrogen sulphide and off-flavour formation are also better understood and can be addressed through improved design and standard operating procedures. However, the complexity of these engineered ecosystems remains, as physical, biological and chemical processes, including their various interactions, must be considered. Standard operating procedures are also improving over time as the knowledge base improves and software solutions assist operators. The next level of operational control is expected to come from the application of AI and computer vision, which will be able to detect key performance indicators such as growth, survival and feed conversion in real time and analyse the complex data to automate predictive modelling.