Bracken (Pteridium aquilinum) is native to this country, but has become a major weed of marginal and hill land throughout western and northern Britain. Estimates suggest that the plant now occupies 3000-6700 km<latex>$^2$</latex> and is spreading at 1-3% per annum. It is a serious weed for several reasons. It causes direct loss of grazing land, is poisonous to stock, and makes shepherding very difficult. It also acts as a reservoir for sheep ticks, causing problems for farmers and managers of grouse moors (ticks transmit louping ill to grouse chicks). The plant is carcinogenic, and has been implicated in higher than average incidences of cancers in people living in brackeninfested areas. Finally, invasion by the plant leads to a loss of plant and animal communities that conservationists regard as more desirable than dense stands of bracken, for example heather moorland. Total costs to agriculture caused by bracken invasion are unknown, but probably run into several million pounds a year. The plant can be controlled by herbicides, or by cutting and rolling, but these methods are often too expensive or too labour intensive for use in many upland areas. One solution may therefore be biological control, although this has rarely been attempted against native plants anywhere in the world. This paper explains why biological control of bracken by using exotic insects from the Southern Hemisphere has a reasonable chance of success. Several potential control agents have now been found on bracken growing in temperate parts of South Africa. They include two moths: Conservula cinisigna, a folivorous noctuid, and one or more species of Panotima, pyralids that first mine the pinnae, and then bore into the rachis. Both appear to be bracken-specific. Their biologies, and those of other possible control agents are described, and constraints and problems encountered in trying to rear them under quarantine conditions are outlined. Over and above the biological and technical problems that have been encountered, and now largely overcome, are a host of political, legal, environmental and socio-economic problems that must be confronted before biological control of bracken in Britain can be attempted. The ecological and economic consequences of controlling bracken biologically need to be carefully weighed against the effects of its continuing spread, and against alternative solutions, for example, harvesting for biomass or control via markedly increased use of herbicides in upland areas.