A comprehensive overview of the basic principles of radar polarimetry is presented. The relevant fundamental field equations are first provided. The importance of the propagation and scattering behavior in various frequency bands, the electrodynamic foundations such as Maxwell's equations, the Helmholtz vector wave equation and especially the fundamental laws of polarization are presented in the first section. Main poins are the polarization Ellipse, the polarization ratio, the Stokes Parameter, and the Stokes and Jones vector formalisms as well as its presentation on the Poincare sphere and on relavent map projections. The Polarization Fork descriptor and the associated van Zyl polarimetric power density and Agrawal polarimetric phase correlation signatures are introduced also in order to make understandable the polarization state formulations of electromagnetic waves in the frequency domain. The different relevant matrices, the respective terms like Jones Matrix, S-matrix, Muller M-matrix, Kennaugh K-matrix, etc. and its interconnections are defined and described together with change determined for the coherent and partially coherent cases, respectively. Concludes with worked examples and references for further reading.