Algae are a diverse and dynamic group of plants which have potential to be developed for a number of applications including agricultural, industrial and medicinal use. Our research has focussed on a number of these biotechnological aspects. The southern African kelp Ecklonia maxima is made into a seaweed concentrate which is used as an plant stimulant. It improves the vigour, yield and disease resistance in a wide range of crops. The active ingredients are thought to be plant growth regulators, mainly cytokinins and auxins. Unicellular soil algae (both Cyanophyta and Chlorophyta) also have an agricultural application being used as soil conditioners, and in the case of Cyanophyta a source of nitrogen. An added incentive to use these algae is that cytokinins and other secondary products have been identified in a number of these unicellular algae which may benefit the crops. Another application for macroalgae is the remediation of metal contaminated waste water. The cell walls of kelps are able to rapidly and effectively bind the heavy metal ions, lowering the ion concentration in solution in small-scale laboratory experiments. These ions can be recovered using a weak acid solution and the seaweed biosorbent regenerated for reuse. This has the potential to be developed into a low cost, effective waste water treatment. The medicinal properties of some South African macroalgae were investigated in various bioassays. The most promising results were obtained in the Cox-1 antiinflammatory, cytotoxic and antimicrobial assays.