Through Wall Imaging radars enable users from the areas of the police, military or disaster control to identify textures behind the wall that is urgently needed for later actions from a safe distance. A major challenge of the Through Wall Imaging technology is to get rid of the negative parts of the backscattered data so that the image can be interpreted correctly and unambiguously. An important factor affecting the image degradation is the first wall structure, which leads to distortion in the signal when the electromagnetic waves pass through the wall. In order to reconstruct a measured radar image of a complex target scene behind the wall as accurately as possible, it is mandatory to know the specific composition of the wall. The paper will give an overview of an investigation of wall structure determination by using microwave radiation including the theory of wave propagation in layered media and simulation results for radar range signatures of artificial and well defined wall structures. Furthermore measurement results of radar range signatures of real materials like calcium silicate brick and hollow brick as well as radar range signatures of layered materials in the frequency range of 1 to 8 GHz and 2 to 18 GHz are presented.