Scientists at JPL measure radiation emitted in the far-infrared (or terahertz) region of the spectrum to study the history of the universe and the Earth's atmosphere. The most commonly used detection technique is frequency mixing between that emission and a local oscillator operating at a frequency close to the emission line. The local oscillators available today, such as optically pumped terahertz lasers or frequency-multiplied millimeter-wave sources, suffer from a number of shortcomings including limited tuning range, inadequate wavelength coverage, large size, and low wallplug efficiency. Physical Sciences Inc. has recently demonstrated a novel, tunable THz laser source based on an external cavity stabilized THz Quantum Cascade Laser with discontinuous tuning over a 30 GHz band at 147 wavenumbers. In the proposed Phase I program, we will use an available 158 wavenumber THz QCL to optimize the external cavity design for continuous, mode-hop free tuning over an expected range of 100 GHz with a target laser linewidth below 1 MHz. Such a laser has never been previously demonstrated. During Phase II we will prove practicality by optimizing the properties of the laser, packaging it into a form suitable for terahertz emission experiments, and delivering it to JPL.