Soil permittivity is a measure of the water content (soil moisture) in the soil and characterized by the metric of the dielectric constant of the soil. Soil moisture influences emission, absorption and propagation of microwave electromagnetic energy. Moisture decreases the ‘emissivity’ of soil, and thereby affects microwave radiation emitted from Earth’s surface. Dry soil has a low dielectric constant and low radar reflectivity. Moist and partially frozen solis have intermediate values. The higher the soil water content, the lower the radar signal penetration into the soil. In situ measurements of soil permittivity are a prerequisite for the calibration and validation of synthetic aperture radar (SAR) soil moisture retrieval algorithms. Soil moisture is a key variable in the hydrologic cycle and is recognized as an Essential Climate Variable (ECV).