With the advent of micro-electro-mechanical systems (MEMS) and the associated fabrication technologies, there is a renewed impetus in thermally-driven flows and its applications. Typical examples include the thermal transpiration flow, radiometric flows and Knudsen pumps. Since radiometers could work as Knudsen pumps for gas transport without any moving parts, they not only can be developed as radiometric actuators for spacecraft attitude control system, but also can be used for a micro-scale gas chromatography and gas separation applications. Due to its important applications in industry, both experimental investigations and numerical simulations have been carried out.