Pesticide research has provided compounds of progressively increased activity, and recently discovered insecticides, fungicides and herbicides are outstandingly potent. Consideration of the practical use of such chemical agents suggests that greater attention should now be given to methods of application and to the physico-chemical properties that determine redistribution and biological availability following release. Effective economical control agents are not yet available for several categories of damaging organism including viruses, bacteria and certain soil-borne pests and diseases. Analysis of the reasons why these categories have proved so refractory suggests that alternative chemical approaches, such as treatments that modify the host plant, and additional properties such as mobility in plants should be investigated. Such analysis also emphasizes the need to consider chemical approaches together with other methods of control in relation to the life histories and population dynamics of the species concerned. Detailed consideration of this principle can not only yield information about the optimum deployment of chemical agents to ensure their continuing efficacy but also suggest the types of action that would be most appropriate. The potential for further improvements in the materials available and methods of use is substantial, but how far this potential is realized will depend as much on policy and economic considerations as scientific factors.