This episode is a conversation with Ruud Hoogeveen from the Netherlands Institute for Space Research about satellite-based Earth observation, and primarily about measuring the concentration of gases such as CO2 or Ozone from space. We talk about the effects of these gases on the atmosphere, how the sensors work in principle, and about the history and evolution of the sensors over the various missions. We conclude with a look on detecting and measuring aerosols and at the future challenges and current research for satellite-based earth observation.