Recent advances in emission and detection of terahertz radiation using two-dimensional (2D) electron systems in III-V semiconductors and graphene. 2D plasmon resonance is first presented to demonstrate intense broadband terahertz emission and detection from InGaP/InGaAs/GaAs and InAlAs/InGaAs/InP material systems. The device structure is based on a high-electron mobility transistor and incorporates the author's original asymmetrically interdigitated dual-grating gates. Second topic focuses on graphene, a monolayer carbon-atomic honeycomb lattice crystal, exhibiting peculiar carrier transport and optical properties owing to massless and gapless energy spectrum. Theoretical and experimental studies toward the creation of graphene terahertz injection lasers are described.