Ozone plays a key role in the atmosphere and participates in numerous important chemical reactions. Apart from the atmospheric chemistry, ozone is also an important green-house gas, contributing significantly to climate change. The paper is focused on ambient (ground-level, surface) ozone (O3), which is considered, together with aerosol, to be currently the most dangerous ambient air pollutant. The formation and depletion of O3 in the atmosphere, as well as its impacts are explained. Moreover, the O3 monitoring in the Czech Republic, and spatial variability and time trends of its concentrations are presented, and impacts of elevated exposures in the Czech Republic are indicated.