Increasing threats of terrorism and contraband smuggling have led to a growing interest in high-frequency security sensors. Recently, systems combining active and passive microwave sensing into one unit have been proposed to improve reliability in the detection of contraband. However, the capability to measure material properties in real-time at standoff distances of a few meters would greatly enhance the ability of security sensors to classify concealed items. This talk discusses a noninvasive approach to extract the complex permittivity and thickness of hidden or embedded dielectrics using multi-mode (active-passive) sensing. The design and operation of two types of microwave sensors are discussed—radar (active) and radiometer (passive), both operating in the K-band (18-26 GHz). Critical factors that degrade the performance of both systems often involve imperfections in the RF front-end as well as background clutter. To mitigate these issues, the following calibration techniques are presented to: (i) correct RF front-end distortion in frequency-modulated continuous wave radars, (ii) correct mismatch and temperature-dependent insertion loss contributions in the RF front-end of radiometers, and (iii) correct background noise for radiometers in indoor applications. Electromagnetic models to predict the radar and radiometric responses of multi-layer dielectric targets are investigated and experimentally verified using dielectric stacks backed by a simplified human phantom (warm water). The models are then used to extract the dielectric constant, loss tangent, and thickness of an embedded layer within the stack with 5% accuracy. While presented from a security standpoint, the techniques presented in this talk may also be used in other applications like remote sensing, detection of buried fires, and food safety inspections.