Metals reflect, plastics transmit, and water absorbs terahertz-frequency electromagnetic radiation. Such diverse responses open up a vast range of applications for terahertz materials spanning art, science, engineering, and medicine. The three main components of terahertz devices are sources, detectors, and the intervening optics. Sources include solid-state emitters, typically involving in their operation either the lattice (nonlinear optics) or the charge carriers (transient dipoles). Quantum cascade lasers, built of multiple semiconductor layers, represent a rapidly developing solid-state terahertz source. Detectors typically depend on either the crystal lattice (electro-optical detection) or the charge carrier reservoir (electronic detection) being sensitive to terahertz radiation. Terahertz components encompass metal-coated mirrors, plastic (machined, molded, or three-dimensional (3-D) printed) lenses, and waveguides, filters, and polarizers of many different materials and designs. An emerging class of components are the terahertz metamaterials .