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We summarize our progress made on developing widely tunable monochromatic terahertz sources. They have been implemented based on difference-frequency generation (DFG) in GaSe, ZnGeP, and GaP crystals. Using a GaSe crystal, the output wavelength was tuned continuously in the range of 66.5 to 5664 m (from 150 to 1.77 cm with the peak power reaching 389 W. Such a high peak power corresponds to a conversion efficiency of 0.1% (a photon conversion efficiency of 19%). A further optimization on the terahertz beam parameter may result in higher output powers and conversion efficiencies. Our experimental results indicate that within the range of 100-250 mum, the output peak powers were higher than 100 W. On the other hand, based on DFG in a ZnGeP crystal, the output wavelength was generated to be tunable in the ranges of 83.1-1642 mum and 80.2-1416 mum for two phase-matching configurations. The output power reached 134 W. Using a GaP crystal, the output wavelength was tuned in the range of 71.1-2830 m, whereas the highest peak power reached 15.6 W. GaP offers an advantage for tuning the output wavelength compared with GaSe and ZnGeP since crystal rotation is no longer required. Instead, one just needs to tune the wavelength of one mixing beam within a narrow bandwidth of 15.3 nm. Based on power scaling, a shoe box-sized tunable terahertz source is feasible. We also review our recent results obtained following the investigation of backward DFG and feasibility studies on backward parametric oscillation. The terahertz radiations produced by DFG have pulse durations of about 5 ns, wide tuning ranges, and narrow linewidths, which are quite different from the broadband terahertz pulses. We also describe a few important applications that were realized by taking advantage of the wide tuning range and narrow linewidths of terahertz pulses such as chemical sensing and differentiation of isotopic variants by measuring the rotational spectra of gases and terahertz imaging.