The terahertz (THz) region of the electromagnetic spectrum, between the infrared and microwave range, has historically suffered from a lack of sources and detectors, leading to the phrase ‘terahertz gap’ to be coined. In recent years however there have been a number of sources and detectors developed in this region. Two of these new technologies are THz time-domain spectroscopy (TDS) and THz quantum cascade lasers (QCL).In this project we seek to combine these technologies and phase-lock the THz QCL in the time domain to a TDS system. This will allow the temporal resolution and coherent detection of the electric field output of the QCL and will permit investigations into the fundamental laser processes of the ultra-fast gain build-up, saturation and recovery times over time-scales of pico-seconds. The first stage of the project will demonstrate the effect of phase locking by injection of a THz pulse into the QCL and forcing the latter to show laser action at a known phase. In the latter stages of the project, the process of phase locking will be used to unambiguously demonstrate mode-locking and the generation of THz pulses from a QCL in the time domain. As well as directly measuring the electric field dynamics of a mode-locked laser, the essential parameters governing mode-locking such as gain lifetimes will be investigated.Finally, as well as insights into fundamental physics of ultra-fast laser field build-up, this project will have several technological benefits. The resulting system will produce short, high-power, phase-resolved pulses of THz radiation that are currently only available at large scale facilities. These generated ultra-fast pulses can be designed in terms of duration, tunability and bandwidth using the unique QCL property of bandstructure engineering. As such, this project will have far reaching benefits for scientific research and the European technology industry.