Recent progresses made by authors on monochromatic and tunable terahertz(THz)generation based on nonlinear optics are reviewed,including THz parametric oscillation(TPO)and difference frequency generation(DFG).From the technical point of view,we develop extra-and intra-cavity surface-emitted TPO,as well as DFG with QPM-GaAs crystal.From the point of view of mechanism,Cherenkov phase-matching is comprehensively investigated in both bulk crystal and planar waveguide.A novel scheme for cascading enhanced Cherenkov DFG in waveguide is proposed.From the point of view of material,organic crystal4-N,N-dimethylamino-4’-N’-methyl-stibazolium tosylate(DAST)is utilized as the nonlinear medium. Recent progresses made by authors on monochromatic and tunable terahertz (THz) generation based on nonlinear optics are reviewed, including THz parametric oscillation (TPO) and difference frequency generation (DFG) .From the technical point of view, we develop extra- and intra- cavity surface-emitted TPO, as well as DFG with QPM-GaAs crystal. From the point of view of mechanism, Cherenkov phase-matching is comprehensively investigated in both bulk and planar waveguide. A novel scheme for cascading enhanced Cherenkov DFG in waveguide is proposed.From the point of view of material, organic crystal4-N, N-dimethylamino-4’-N’-methyl- stibazolium tosylate (DAST) is utilized as the nonlinear medium.