The important status of the River Tweed and its biology are detailed, included the past and present day biologicakl investigations/monitoring undertraken by the Tweed River Purification Board now under the aegis of the Scottish Environment Protection Agency (SEPA). The nature of the Tweed's biology is examined in terms of its algal, macrophyte, invertebrate adn fish populatons with some descriptions of their natural status. In addition, major historical and present day water quality problems are mentioned in terms of the impacts that they have had upon these biological communities.