The review gives a general outline of macro- and microalgal biotechnology. The main methods by which algae are cultivated and harvested are described. The first section deals with the environmental factors affecting mass culture and the principles governing the design and operation of mass cultivation systems. The second section gives the main current and potential uses of algae: in wastewater treatment, a source of food and feed, an energy source, and in the production of common and fine chemicals, such as polysaccharides, lipids, glycerol, pigments, and enzymes. Pharmaceutical uses of algae are described, and their potential as a source of novel biologically-active compounds is discussed. Future developments and the great potential of algae are considered.