Ron Smith, teaching Yale's Atmosphere, Ocean and Environmental Change course, devotes this lecture to stratospheric ozone and how it shields the planet from ultraviolet radiation. He works through the terminology of the ozone layer, the Dobson Unit used to measure ozone concentration, and the chemical cycle by which ozone forms and breaks down in the stratosphere. The lecture explains how chlorine atoms, mainly from chlorofluorocarbons, destroy ozone molecules, and traces CFC emissions rising after 1960 and climbing through the 1990s. Smith covers the discovery and mechanics of the Antarctic ozone hole before ending with the 1987 Montreal Protocol, which banned CFC emissions starting in 1994 and brought them to near zero today. The talk moves chapter by chapter through definitions, measurement, causes, and policy response, giving a full account of one of the clearest cases of measured environmental damage followed by an international fix.