Technical Abstract: Herbicides are small molecules that inhibit specific molecular target sites within plant biochemical pathways and/or physiological processes. Inhibition of these sites often has catastrophic consequences that are lethal to plants. The affinity of these compounds for their respective target sites makes them useful tools to study and dissect the intricacies of plant biochemical and physiological processes. For instance, elucidation of the photosynthetic electron transport chain was achieved in part by use of herbicides such as terbutryn and paraquat, which act on photosystem II and I, respectively, as physiological probes. Work stemming from the discovery of the binding site of PS II-inhibiting herbicides was ultimately awarded the Nobel Prize in 1988. Though not as prestigious as the seminal work on photosynthesis, our knowledge of many other plant processes expanded significantly through the ingenious use of inhibitors as molecular probes. Examples of the critical role played by herbicides in expanding our understanding of fundamental aspects of the synthesis of porphyrins and the non-mevalonate pathway, the evolution of acetyl-coenzyme A carboxylase, cell wall physiology, the functions of microtubules and the cell cycle, the role of auxin and cyanide, the importance of subcellular protein targeting, and the development of selectable markers will be given.