The hydroxyl radical (OH) is an oxidizing molecule that has a major impact on the distribution and concentrations of pollutants and greenhouse gases in Earth's atmosphere. It is the most common oxidant in the troposphere, the lowest portion of the atmosphere. Understanding OH variability is important to evaluating human impacts on the atmosphere and climate. Measuring global OH distribution and variability, however, has been difficult, in part because the molecule has a lifetime of less than one second. On page 67 of this issue, Montzka et al. (1) use an indirect method to present a global estimate of OH's interannual variability that is consistent with past model-based estimates (2–5). This could be an important step toward understanding OH's role in the atmosphere, including its role in influencing concentrations of methane (CH4), a gas important in considerations of global warming.