Book excerpt: Developments in the understanding of herbicide activity and toxicology have expanded tremendously in the past fifteen years. Research on the mechanism of action of most major classes of herbicide chemistry has provided scientists with excellent insight into enzyme targets. More recently, developments in molecular biology have provided information about herbicide action at the genetic level. Less well understood are the toxicological aspects of herbicide activity that culminate in plant injury or death. Toxicology, Biochemistry and Molecular Biology of Herbicide Activity is a review of the recent literature on most of the major classes of herbicide chemistry in commercial use. The chapters include information about different aspects of herbicide activity related to photosynthesis, inhibition of amino acid biosynthesis, disruption of cell division and microtubule assembly, activity of phytohormone (auxin) mimics, inhibition of fatty acid biosynthesis and some developments in the understanding of herbicide resistance.