This paper gives an overview to the fundamental principles, key components, system design and technical advantages of quasi-optical technology, particularly for its applications in the millimeter and terahertz systems in fields of remote sensing, radioastronomy, and imaging. A perspective to the state-of-the-art and future development of quasi-optical technology has been given. Quasi-optical technology is a set of system design and analysis techniques based on Gaussian beam propagation in free space. Systems based on quasi-optical technology bears advantages of high-power handling capability, multi-polarization, low transmission loss, and high compactness. In the future, quasi-optical technology will be seeking more applications in these areas and further development can be highly envisaged.