The interaction of intense laser radiation with pre-ionized plasma can lead to various nonlinear effects such as wakefield generation for particle acceleration, terahertz (THz) radiation generation, harmonic generation, self-focusing, and laser fusion schemes. Traditional sources for terahertz radiation generation are of low intensity due to material breakdown limit. Terahertz radiation sources based on interaction of intense lasers with plasma can prove to be a viable replacement of conventional systems for applications that require high intensities. The present chapter deals with theoretical studies of terahertz radiation generation by interaction of intense circularly polarized laser pulses with homogeneous and inhomogeneous plasmas.