Chemistry has once again embraced the study of enzyme mechanism as a core discipline. Chemists are uniquely able to contribute to the analysis of enzymes through their understanding of the reactivity of atoms. In this tutorial review for the Corday-Morgan medal, I will concentrate on the work from my lab over the past six years. I discuss enzymes which transform carbohydrates and incorporate halogens. The tutorial review will emphasise the strengths and limitations of structural biology as a means to deducing the chemical mechanism.