Algae are utilized diversely in aquaculture, but their main applications are related to nutrition. They are used in toto, as a sole component or as a food additive to supply basic nutrients, color the flesh of salmonids or for other biological activities. The need for nutritional sources safer than traditional animal products has renewed interest in plants in general and algae in particular. This report deals principally with the nutritional role of microalgae in aquaculture.The larvae of molluscs, echinoderms and crustaceans as well as the live prey of some fish larvae feed on microalgae. Though attempts have been made to substitute inert particles for these micro-organisms which are difficult to produce, concentrate and store, only shrimp and live prey for fish will accept inert food, and only shrimp accept it fully. Several studies have confirmed that a live, multi-specific, low-bacteria microalgal biomass remains essential for shellfish hatcheries. Major advances are expected from new production system designs and operations, from batch-run open tanks to more sophisticated continuously run and closed loop reactors. Studies are underway to simplify hatchery operations by replacing biomass produced on-site with run-times by that produced and preserved elsewhere. Although still promising, they have not given rise, so far, to any application for molluscs. Other applications of microalgae in aquaculture, from green water to making salmon flesh pinker, are examined. Whether produced on or off-site, there remains the question of cost effectiveness of microalgal production systems. This can only be achieved by substantial upscaling and improved quality control.