Aside from active radar remote sensing techniques such as SAR, it is also possible to measure the natural microwave emission of the Earth. This lesson deals with the principles of “Passive Microwaves”. It introduces the thermal radiation theory as well as concepts like “brightness temperature”. The dielectric and emissive properties of the Earth’s surface are explained. Furthermore, this lesson presents existing missions and sensors to measure passive microwave radiation. Several applications over land, oceans and atmosphere are displayed.