Two important groups of herbicides, the cyclohexanediones (CHD) and aryloxyphenoxypropanoates (AOPP), inhibit the plastidic enzyme acetyl-CoA carboxylase (ACCase; E.C. 6.4.1.2). Representative compounds in these groups are shown in Fig. 1. A third class of inhibitor, based on a hybrid cyclic triketone structure, shows similar herbicidal activity (Rendina et al. 1995), but has not been developed commercially. CHD and AOPP herbicides are used to control a wide selection of grass weeds in both monocot and dicot crops. The basis of selectivity differs between dicot and grasses: in dicots, tolerance is based on the inherent insensitivity of dicot ACCase to these herbicides, whereas in certain cereal crops selectivity is based on higher rates of herbicide detoxification in the crop species (Devine and Shimabukuro 1994). This chapter will review the general activity of these herbicides, the biochemistry of the target enzyme, and the molecular basis of resistance in crops and weeds.