Microwave radiations are localized on the electromagnetic spectrum between radio frequencies and infrared. The dimensional aspect is of crucial importance in understanding the complex interaction phenomena occurring when a microwave beam propagates in biological structures. As a matter of fact, due to comparable orders of magnitude between the wavelength and the structure dimensions, the predominant interaction mechanism is diffraction. The microwave beam/biological target interaction is strongly dependent on the tissue characteristic called the complex permittivity or, equivalently, dielectric constant. The chapter presents the integral formulation that provides the basis for considering non-invasive thermometry as an electromagnetic inverse scattering problem. The non-invasive thermometry problem consists of retrieving the temperature distribution in the target from its perturbation of the incident field, namely from its diffracted field. A prototype imaging system for active microwave tomography using a circular array of antennas has been developed.