This paper presents the development of ultrathin, broadband, and highly efficient metamaterial-based terahertz (THz) polarization converters that can rotate linear polarization states into their orthogonal counterparts. The authors demonstrate two main designs: a reflection-based converter using a metal cut-wire array and a metal ground plane, and a transmission-based converter using a metal grating. Both designs achieve high efficiency and broadband operation, with the reflection-based converter showing cross-polarized reflection exceeding 50% of the incident power between 0.7 and 1.9 THz, and the transmission-based converter achieving a conversion efficiency exceeding 50% from 0.52 to 1.82 THz. Additionally, the paper discusses the creation of metamaterial structures capable of realizing near-perfect anomalous refraction, which is achieved by eliminating ordinary components in the reflection/refraction process. The work opens new opportunities for high-performance photonic devices and enables emerging functionalities in the challenging THz frequency regime.This paper presents the development of ultrathin, broadband, and highly efficient metamaterial-based terahertz (THz) polarization converters that can rotate linear polarization states into their orthogonal counterparts. The authors demonstrate two main designs: a reflection-based converter using a metal cut-wire array and a metal ground plane, and a transmission-based converter using a metal grating. Both designs achieve high efficiency and broadband operation, with the reflection-based converter showing cross-polarized reflection exceeding 50% of the incident power between 0.7 and 1.9 THz, and the transmission-based converter achieving a conversion efficiency exceeding 50% from 0.52 to 1.82 THz. Additionally, the paper discusses the creation of metamaterial structures capable of realizing near-perfect anomalous refraction, which is achieved by eliminating ordinary components in the reflection/refraction process. The work opens new opportunities for high-performance photonic devices and enables emerging functionalities in the challenging THz frequency regime.