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The different approaches to control of weeds by biological methods are reviewed. The methods depend upon the techniques used and the control desired. These approaches are classified as follows: the classical or inoculative method, based on the introduction of host-specific exotic natural enemies adapted to exotic weeds; the inundative or augmentative method based on the mass production and release of native natural enemies usually against native weeds; the conservative method based on reducing numbers of native parasites, predators and diseases of native phytophages that feed on native plants; and the broad-spectrum method based on the artificial manipulation of the natural enemy population so that the level of attack on the weed is restricted to achieve the desired level of control. Examples of each of these different methods are presented and the advantages and disadvantages are discussed. The greatest development for the future potential usage of biological control will be the use of biotic agents in an integrated approach to weed management.