The capability and use of satellites to monitor Earth’s atmospheric composition has increased exponentially in the last decades, together with the rapid increase of the need of having accurate information to understand and mitigate air pollution and climate change effects. Trace gases like carbon dioxide (CO2), methane (CH4), ozone (O3), nitrogen oxides (NOx = NO + NO2) even though they constitute a small part of atmospheric composition, they play an important role in the Earth’s climate  and they also affect the quality of the air that we breathe. Satellite instruments like GOME, SCIAMACHY, OMI and GOSAT have been monitoring these gases for long time periods. TROPOMI satellite instrument provides since October 2017  measurements of multiple trace gases at an unprecedented spatial and temporal resolution. This allows us to know atmospheric composition from multiple perspectives: from long term records to individual pollution events in cities or power plants.  In this seminar I will explain how satellites are capable of measuring gases from the light reflected by the Earth’s atmosphere, and how are these measurements used to monitor the concentration and emissions of gases like NO2, CO and CH4.