Understanding photo-physics in low-dimensional materials, including charge generation, cooling, transport and recombination, is essential for both fundamental science and optoelectronic applications. Many-body effects, e.g., how photogenerated electrons interact with other charge carriers (holes or electrons themselves), phonons, and defects, are known to play a critical role in dictating materials’ optical and electrical properties; reducing the material dimensionality can substantially enhance such interactions due to combined effects of spatial confinement, weakened electronic screening and reduced (electronic or phononic) density of states. The thesis is dedicated to understanding the ultrafast charge carrier dynamics in one- and two-dimensional (1D/2D) quantum materials by terahertz (THz) spectroscopy. First, we present an overview of the basic many-body interactions in quantum materials, including electron-electron scattering (e.g., for carrier heating in metals and impact ionization in semiconductors), carrier-phonon interaction (“polaron” effect), and electron-hole interaction (“exciton” effect). Then we introduce the main experimental technique (THz spectroscopy), and details of the data analysis for inferring the optical and charge transport properties in materials of interest. Finally we summarize key results of the thesis with a primary focus on ultrafast carrier-carrier scattering and electron-phonon in 2D materials (e.g., layered MoTe2 and MXenes), and electron-defect scattering in 1D carbon nanotubes. Overall, our findings elucidate the ultrafast dynamics and transport properties of photogenerated charge carriers subjected to many-body effects in quantum materials, providing new insights into the fundamental photophysical processes relevant to (opto-) electronic applications.