Pyrazinamide (PZA) is an unusual antituberculosis drug because its bactericidal activity increases as bacterial metabolism decreases, for instance in models of bacterial persistence, from the action of energy inhibitors, during hypoxic incubation and at low incubation temperatures. These characteristics make PZA particularly effective in killing persistent bacteria that other antibacterials fail to kill, and so make it an essential drug to be included in any current or new drug combination. This behaviour is explained by the Zhang model of PZA in which PZA is first deaminated to the active form of the drug pyrazinoic acid (POA) which enters the cell by passive pH-dependent diffusion but can only leave through an inefficient efflux pump which requires energy. If insufficient energy is available to drive its efflux, POA accumulates within the cell and is lethal. Weak acids, iron, oxidative stress, UV and DNA damage also increase PZA activity. Resistance to PZA is almost always due to mutations that occur widely distributed within the pncA gene that codes for pyrazinamidase involved in the conversion of the prodrug PZA to POA. We suggest increasing the cut-off concentration defining resistance in pH 6.0 medium and the use of the Brander test with high concentrations of nicotinamide in medium at normal pH for phenotype-based PZA susceptibility testing. Sequencing of pncA for mutations represents a rapid molecular test for detecting PZA resistance and avoids the problem of conventional phenotype-based PZA susceptibility testing.