Herbicides are used to control weeds and are usually targeted to processes and target sites that are specific to plants. As a result, most herbicides are not acutely toxic to fish. Exceptions to this general rule are uncouplers of oxidative phosphorylation and some herbicides that interfere with cell division. Chronic and sublethal effects have been studied for some herbicides, but fewer data are available for these effects than for acute effects. The sublethal effects of herbicides that have been studied include reproduction, stress, olfaction, and behavior. Although some of these responses have been observed in fish exposed to herbicides, these have either been observed at large concentrations that would be rarely found in surface waters inhabited by fish or, as in the case of behavior and olfaction, have not been linked to ecologically relevant responses on survival, growth, development, and reproduction. As with all pesticides, herbicides may have indirect effects in fish. These effects are mediated by herbicide-induced changes in food webs or in the physical environment. Indirect effects can only occur if direct effects occur first and would be mediated by the killing of plants by herbicides. Although this indirect effect might occur when plants are controlled by direct treatment of surface waters, indirect effects appear to be unlikely to result from use of herbicides in terrestrial systems as runoff concentrations in surface waters are, for the most part, very much less than those that could directly affect plants.