To single technique has provided an exact means for ascertaining now herbicides modify or inhibit plant growth and development. Electron microscopic studies also have limitations: for example, the amount of tissue that can be examined even in one’s lifetime is exceedingly small. Electron microscopic studies are essentially limited to changes induced by the herbicides in structural components of the cell. The structure of the tissue observed represents only that which is preserved at the time of fixation and an adequate sampling of the normal range of variation in the tissue is difficult. Also, some fixatives (potassium permanganate) destroy organelles such as ribosomes. Although ultrastructural examination may not ultimately provide an exact diagnostic tool, such studies correlated with biochemical investigations provide insight into possible mechanisms of herbicidal action that the latter alone could not provide. Thus, it is the purpose of this review to compare and evaluate the reported effects of various herbicides on plant ultrastructure with particular emphasis on those aspects which differ significantly from normal senescence. Further, an attempt will be made to establish correlations, wherever possible, between herbicidal-induced alterations in ultrastructure with known changes in physiology and growth. The danger that such correlations may be between secondary effects which can mask the primary treatment effects is recognized.