Self-cleaning processes in the atmosphere occur via oxidation processes, dominated by OH radicals. OH measurements taken in different environments, from remote marine locations, tropical and boreal forests as well as over Europe, and at different altitudes, illustrate our current understanding of HOx chemistry. While in clean air measured OH matches expectations, more polluted air, and also air with biogenic emissions, often shows enhancements of OH that indicate unknown additional processes. Convection can transport such air into higher altitudes, significantly alterating the oxidative chemistry in the upper troposphere.