Technical Abstract: Herbicides with similar target sites and modes of action can be grouped to help growers better manage herbicide resistant weed populations by rotating herbicides with different target sites. Many herbicides commonly used in the Pacific Northwest can be grouped into the following categories: acetyl CoA carboxylase (ACCase) inhibitors, acetolactate synthase (ALS) inhibitors, microtubule assembly inhibitors, synthetic auxins (phenoxys, benzoics, picolinic acids), photosystem II inhibitors, glutamine synthetase inhibitors, protoporphyrinogen oxidase (Protox) inhibitors, and chloroacetamides. Herbicides within each group should be rotated with herbicides with different modes of action to delay or prevent buildup of herbicide resistant weed populations. In the Pacific Northwest, Prickly lettuce, Russian thistle, and kochia have developed resistance to herbicides that inhibit the ALS enzyme. Pigweed, common lambsquarters, common groundsel have developed resistance to herbicides that inhibit photosystem I. Wild oat populations have developed resistance to both ACCase and lipid synthesis inhibitors. Continuous use of herbicides with the same mode of action in the crop rotation has contributed to the buildup of these herbicide resistant weed populations.