THz radiation is generated from topological insulators by C. W. Luo, J.-Y. Lin, and co-workers using an 800 nm femtosecond laser. Two-channel free carrier absorption with bulk and surface carriers is indispensable to explaining the strong dependence of THz emission power on the carrier concentration. The surface carriers in topological insulators are significantly better absorbers of THz radiation than bulk carriers at room temperature. Moreover, the characteristics of THz emission provide valuable information on page 804 regarding the fundamental properties of Dirac fermions.