For decades after its introduction, the mechanisms of action of the front-line antituberculosis therapeutic agent isoniazid (INH) remained unclear. Recent developments have shown that peroxidative activation of isoniazid by the mycobacterial enzyme KatG generates reactive species that form adducts with NAD+ and NADP+ that are potent inhibitors of lipid and nucleic acid biosynthetic enzymes. A direct role for some isoniazid-derived reactive species, such as nitric oxide, in inhibiting mycobacterial metabolic enzymes has also been shown. The concerted effects of these activities – inhibition of cell wall lipid synthesis, depletion of nucleic acid pools and metabolic depression – drive the exquisite potency and selectivity of this agent. To understand INH action and resistance fully, a synthesis of knowledge is required from multiple separate lines of research – including molecular genetic approaches, in vitro biochemical studies and free radical chemistry – which is the intent of this review.