In this chapter, the radar principle is reviewed, and the importance of polarimetric information is explained. The basic principles of radar polarimetry, the receiving antenna voltage starting from the reciprocity theorem, Sinclair scattering matrix, and the receiving power are defined. An important point is how the receiving power is changed by polarization and scattering matrix. Since receiving power changes according to polarization states of the transmitter and receiver, it becomes possible to choose optimum polarization states for various applications. Using a specific polarization state, we can extend it, for example, to obtain characteristic polarization states of target, polarimetric filtering for clutter reduction, and target contrast enhancement. Contrast enhancement by the polarization state is a very promising technique for detection and classification and identification of targets in severely cluttered environments. This superb technique can be accomplished by polarimetric radar only.