Auxinic herbicides are widely used for control of broadleaf weeds in cerealcrops and turfgrass. These herbicides are structurally similar to thenatural plant hormone auxin, and induce several of the same physiologicaland biochemical responses at low concentrations. After several decades ofresearch to understand the auxin signal transduction pathway, the receptorsfor auxin binding and resultant biochemical and physiological responses haverecently been discovered in plants. However, the precise mode of action forthe auxinic herbicides is not completely understood despite their extensiveuse in agriculture for over six decades. Auxinic herbicide-resistant weedbiotypes offer excellent model species for uncovering the mode of action aswell as resistance to these compounds. Compared with other herbicidefamilies, the incidence of resistance to auxinic herbicides is relativelylow, with only 29 auxinic herbicide-resistant weed species discovered todate. The relatively low incidence of resistance to auxinic herbicides hasbeen attributed to the presence of rare alleles imparting resistance innatural weed populations, the potential for fitness penalties due tomutations conferring resistance in weeds, and the complex mode of action ofauxinic herbicides in sensitive dicot plants. This review discusses recentadvances in the auxin signal transduction pathway and its relation toauxinic herbicide mode of action. Furthermore, comprehensive informationabout the genetics and inheritance of auxinic herbicide resistance and casestudies examining mechanisms of resistance in auxinic herbicide-resistantbroadleaf weed biotypes are provided. Within the context of recent findingspertaining to auxin biology and mechanisms of resistance to auxinicherbicides, agronomic implications of the evolution of resistance to theseherbicides are discussed in light of new auxinic herbicide-resistant cropsthat will be commercialized in the near future.