Algae are utilized diversely in aquaculture, but theirmain applications are related to nutrition. They areused in toto, as a sole component or as a foodadditive to supply basic nutrients, color the flesh ofsalmonids or for other biological activities. The needfor nutritional sources safer than traditional animalproducts has renewed interest in plants in general andalgae in particular. This report deals principallywith the nutritional role of microalgae inaquaculture.The larvae of molluscs, echinoderms andcrustaceans as well as the live prey of some fishlarvae feed on microalgae. Though attempts have beenmade to substitute inert particles for thesemicro-organisms which are difficult to produce,concentrate and store, only shrimp and live prey forfish will accept inert food, and only shrimp accept itfully. Several studies have confirmed that a live,multi-specific, low-bacteria microalgal biomassremains essential for shellfish hatcheries. Majoradvances are expected from new production systemdesigns and operations, from batch-run open tanks tomore sophisticated continuously run and closed loopreactors. Studies are underway to simplify hatcheryoperations by replacing biomass produced on-site withrun-times by that produced and preserved elsewhere.Although still promising, they have not given rise, sofar, to any application for molluscs. Otherapplications of microalgae in aquaculture, from greenwater to making salmon flesh pinker, are examined.Whether produced on or off-site, there remains thequestion of cost effectiveness of microalgalproduction systems. This can only be achieved bysubstantial upscaling and improved quality control.