Gear transmission is one of the most important and widely utilized forms of transmission in modern mechanical design. Its applications span various fields, including aerospace, automotive electronics, medical devices, industrial robots, planetary reducers, and gear pumps, among others. However, in certain specialized areas, such as transmission in micro-space, traditional industrial gears often fail to meet the demands due to limitations in their design principles and manufacturing methods. Therefore, it is essential to start from the fundamental principles of gear meshing, break through the traditional theory of spatial conjugate surface meshing, and develop new types of gears to address the needs of special applications like micro-space transmission. Line gear (LG) was proposed to solve the design problem of minimizing gear transmission space. LG is a novel gear mechanism based on the space conjugate curve meshing theory, featuring a high transmission ratio, flexible design, controllable sliding rate, and the capability to function with as few as one gear tooth. Consequently, LG possesses distinct advantages in applications involving micro-miniaturization design, lightweight design and special requirements such as pure rolling meshing transmission, continuous variable shaft angle transmission. On the basis of briefly reviewing the development history and describing the meshing theory of LG, this paper systematically summarizes the key research achievements and development of LG in terms of design theories, manufacturing technologies, associated equipment, and applications. This review also makes some recommendations for future research work on LG.