The paper examines the basic physical principles of the operation of terahertz (THz) polarization converters and discusses the main types of polarizers, including those based on wire girds, films, metamaterials, etc. More sophisticated devices based on polarizers made of stacks of segmented, achromatic, and tunable composite waveplates, including those operating over a wide wavelength range, and on THz polarization filters are considered. The basic principles of their calculation using the Jones formalism are described.