We report our recent progress made on the development of widely-tunable monochromatic THz sources. They have been realized based on difference-frequency generation (DFG) in GaSe, ZnGeP2, and GaP crystals, respectively. Using a GaSe crystal, the output wavelength was tuned continuously in the range from 66.5 µm to 5664 µm (from 150 cm-1 to 1.77 cm-1) with the peak power reaching 389 W. Such a high peak power corresponds to a conversion efficiency of about 0.1% (a photon conversion efficiency of 19%). A further optimization on the THz beam parameter may result in an output peak power of a few kW. Within the range of 100-250 µm the output peak powers were higher than 100 W. We have also investigated THz frequency upconversion in GaSe, ZnGeP2, and GaP crystals. Such a parametric process has a potential for detecting THz pulses at room temperature or just using a thermoelectric cooler. The minimum detectable energy per pulse was measured to be 245 pJ, which corresponds to a noise equivalent energy of 77.5 . A further optimization of the process can reduce this value down to 1 .