Herbicides have been placed in Herbicide Groups based on their sites of action. A major driving force for this classification system is that growers have been encouraged to rotate or mix herbicides from different HG to delay the evolution of herbicide-resistant weeds; because, in theory, all active ingredients within a herbicide group physiologically affect weeds similarly. Although herbicide resistance in weeds has been studied for decades, recent research has demonstrated patterns of cross-resistance are more complex than merely stating a certain weed biotype is resistant to a HG. The objective of this presentation is to highlight the intricacies associated with cross-resistance patterns within HG.