Algae offer special advantages as photosynthetic plants which may be used to provide for the gas exchange requirements of man and for some fraction of his food requirement. Established principles of algal metabolism allow basic specifications. To meet a human oxygen demand of 600 1/day an algal exchanger would take up about 720 1/day of carbon dioxide, produce about 600 g/day of dry algae, and require a minimum of about 800 watt of visible light. Major technical disadvantage arises from need for input power as light and the low values of irradiance at which algae can maintain maximum efficiency. Design criteria require compromise between minimum volume and minimum power requirement. A basic design theory needs to be evolved and a number of technical problems need to be solved before proper evaluation of an algal system can be made.