Presents sample applications of ‘Passive’ DOAS using natural light Light sourcenatural sourcesLightsources for fundamental research, airPollution pollutionAir pollution studies, and trace gas monitoring. This includes direct sunlight Lightsun measurement from the ground, balloons and aircraft, the study of stratospheric and tropospheric trace gases using scattered sunlightLightsun from the ground, monitoring of emissions from point sources, urban areas, and volcanoes, as well as aircraft and satellite DOASSatellite DOAS (S-DOAS) applications. Multi-AXis DOASMulti-Axis DOAS (MAX-DOAS) (MAX-DOAS), as one of the most popular passive DOASPassive DOAS techniques for the measurement of tropospheric species, receives particular attention. The chapter includes an overview of current satellite DOASSatellite DOAS (S-DOAS) capabilities for the monitoring of stratospheric and tropospheric species. Further topics are the growing use of Solar Induced Fluorescence (SIF) from chlorophyl for the remote observation of biological activity, and the determination of photon path lengths (e.g. in clouds) by ‘Reversed DOAS’.