The present review addresses the basic physical principles of terahertz polarization converters functionality and discusses main types of polarizers such as quartz, wire-grid, film-, metamaterial-based, etc. More complicated devices based on set of polarizers such as achromatic and tunable compound waveplates, including those operating over a wide wavelength range, and THz polarizing filters are considered as well. The basic principles of their design based on the Jones formalism are described.