Many pest control approaches are presently promoted as biocontrol, however, in this paper the definition is restricted to ‘the utilization of parasites, predators and pathogens to regulate pest populations’. This covers two approaches against weeds: inundative and classical biocontrol. Inundative control applies an organism (usually a fungal disease) in the manner of a herbicide, and like a herbicide, it is usually marketed by industry. Classical biocontrol establishes an organism (usually an insect) from another region, to give continuing control of the pest and is largely done by governments in the public interest. Both types of biocontrol are regulated by legislation originally designed for other purposes, which is, at best, awkward. Both biocontrol approaches, in the right circumstances, are effective means of weed control with few harmful environmental impacts, but their use is limited by ecological, economic and regulatory constraints. These are illustrated by Canadian examples of a fungal disease for inundative control of round-leaved mallow and the white Amur fish for aquatic weeds. The classical biocontrol examples discussed are St. John's-wort, nodding thistle, leafy spurge and knapweed, as well as some new applications. The steps in classical weed biocontrol are outlined as well as the public involvement needed to develop a biocontrol solution for a weed.