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Chemical control of weeds has increased agricultural productivity, but complete reliance on chemicals has serious drawbacks. These include high cost per acre, decreasing effectiveness, negative effects on plant community diversity, and increased opportunities for environmental contamination. One alternative is biocontrol, the use of biological factors that naturally limit weed populations. Long-term research goals focus on improving our knowledge of the processes that control and limit potential plant pests naturally and to use that knowledge to develop predictable, sustainable, low-cost, biologically-based weed management strategies. This paper reviews the ecological underpinnings of classical biological control of weeds, including basic research on the interaction of natural enemies with native thistles in Nebraska. The fundamentals for developing a biological weed control program are summarized and research in the central and northern Great Plains on applying biological controls to limit introduced noxious rangeland weeds is reviewed. This is followed by a discussion of one of the problems associated with biological control, the potential secondary effects of biocontrol agents on non-target plant species. The evidence suggests that biological control is an ecologically sound, sustainable and economical option for limiting introduced rangeland weeds. Biological control represents an attractive technique for limiting the negative impacts of high weed densities, while not eradicating the introduced weed. Expanding interest in sustainable agricultural systems and more environmentally- friendly pest control and the prohibitive costs of chemical control suggest that biological weed control is likely to become an increasingly important part of rangeland management.