The generation of widely tunable coherent terahertz (THz) frequencies is of great interest for a variety of applications in basic and applied sciences. Broadband THz sources, particularly those based on femtosecond lasers, have already shown much promise in addressing these applications. In this seminar, I will present a narrow bandwidth approach to THz generation by means of difference frequency generation (DFG) between two lasers. Provided that one or both of the laser sources are tunable, the DFG technique offers a pathway to tunable narrowband THz frequencies. Our results based on mixing the output of two seeded optical parametric generators will show narrow-bandwidth and tunable operation from 1.6 to 4.5 THz. I will discuss our current efforts at increasing the DFG conversion efficiency by mixing in photonic band gap crystals, and I will talk about narrow bandwidth applications.