In biological agriculture and horticulture diluted extracts of seaweed are applied to promote growth, prevent pests and diseases and improve the quality of the products. The efficacy of the extracts is probably based upon plant hormones (mainly cytokinins) and trace nutrients present in the extracts. A survey of the recent literature is given concerning the properties of seaweed extracts, methods of application, their effects and putative working mechanisms. The significance of seaweed extracts in biological agriculture and horticulture is evaluated, regarding environmental aspects of production and use.