A number of interactions are possible when electromagnetic energy encounters matter, whether solid, liquid or gas. In Earth Observation there are two main interactions: atmospheric and with target. Atmospheric interaction: In radar remote sensing, atmospheric interactions are limited due to the long wavelengths compared to the size of the atmospheric particles. The fact that microwaves interact with object at least as big as the wavelength is one of the greatest advantages of microwave remote sensing, since at larger wavelengths atmospheric particles are almost transparent to the signal and microwave sensors are independent from the time of day (day or night) and weather conditions. Water clouds can interfere with the radars operating below 2 cm in wavelength. The effects of rain can be generally ignored at wavelengths above 4 cm. For longer wavelengths (above 20 cm), an effect called Faraday rotation caused by the ionosphere, i.e., free charges (electrons) and the Earth’s magnetic field, can lead to a rotation of the polarization plane. Target interaction: The radar interaction with the object is a result of both radar system parameters (frequency, polarization, acquisition geometry) and the physical properties of the object (dielectric constant, i.e., water content; geometrical properties, i.e., the roughness, shape and orientation of the scatterer). Overall, various types of interactions can be distinguished – scattering, diffraction, and reflection – all describing the same process of wave interaction but at different scales.