Herbicides are amazing chemicals. Ideally, we expect herbicides to kill weeds and protect our crops from weed competition, without harming the crop. Fortunately, this is the typical case, but herbicides can damage our crops on occasion. To minimize the risk of herbicide-injured crops, we need to understand how herbicides function and factors that affect their activity. We can use this information to make better decisions regarding the risk of crop injury when selecting or applying herbicides.