Over the last 20 years, the productivity in hatcheries and farms producing fry and smolt of trout and salmon has increased substantially. These land-based farms are mainly situated along the coast and discharge effluent water directly to the sea. Such production is the basis of the recruitment of marine salmon and trout cage farms in Chile, Scotland, Norway and some other temperate countries with a coastline. Similarly, productivity and use of recirculation systems for the production of both seawater and freshwater fish has increased throughout the world. In many cases, the use of recirculation systems has ecological advantages over other technologies, especially those relying on flow through operations. The Norwegian authorities required a minimum flow supply of 1.5 m3 per 100,000 salmon smolt produced annually since the mid-80's. Without oxygenation of the water, the specific flow rate fluctuated between 0.5 and 2.5 L kg-1 min-1 throughout the year. Addition of pure oxygen then significantly reduced the flow requirements, now typically in the range 0.3 – 0.5 L kg-1 min-1. According to another regulation for licensing of hatcheries, the lowest allowable flow in single flow-through systems was 0.3 L kg-1 min-1. Water quality parameters have however been introduced recently as criterion instead of water flow requirements (concentrations of oxygen, carbon dioxide and ammonia). Mainly due to the usage of oxygenation technology, the water consumption at most smolt farms is at present 100 – 200 m3 kg-1 produced fish compared to 1,000 – 1,700 m3 kg-1 some 20 years ago. When the water flow is reduced, there is a build-up of both carbon dioxide and total ammonia, whilst pH is reduced. Both carbon dioxide and pH may become limiting factors when the water flow is decreased due to oxygen injection. There is however a lack of information regarding safe levels of carbon dioxide concentrations for Atlantic salmon smolts. Increased ventilation frequency and reduced growth have been observed in smolts exposed to reduced water flow. Effects observed during long-term experiments with rainbow trout and Atlantic salmon exposed to elevated carbon dioxide in fresh water include reduced growth and feed utilisation and nephrocalcinosis. Recirculation systems allow in the same time to reduce the make up water needs, to control the recirculated water quality and facilitates the treatment of the effluents (lower flow rate and higher concentration). They were developed at commercial scale in several countries and for various marine or freshwater fish species. In this article, some recirculation systems adapted to different fish life stages (from breeders to commercial size fish) and environments (Europe and USA) will be described. Improved feed quality, better feeding control and other factors have strongly reduced the waste production from farms. Since the mid-80's, the mean feed conversion ratio (FCR) in the Norwegian smolt industry as in other developed countries has decreased from 1.5 – 1.8 to less than 1.0 (kg feed kg-1 produced fish) which indicates a halved effluent load of organics and nutrients per kg produced fish. Additionally, many farms have introduced end-of-pipe treatment for solid removal before release to recipient.