Maintaining a clean living and working environment is an important means to ensure people's quality of life. As the height of the building increases, the traditional method of cleaning windows by human workers not only takes a long time, but also brings the risk of falling off. It is necessary to replace human workers with robots for carrying out such heavy-duty and dangerous work. Various window-cleaning robots have been applied in cleaning work for certain actual scenarios. In this paper, the state-of-the-art survey of recent developments on window-cleaning robots is presented. The applications of window-cleaning robots in two main domains of domestic use and high-rise building environments are presented and the corresponding technical requirements are summarized. Afterward, the main techniques of window-cleaning robot development including locomotion mechanisms, adhesion mechanisms, cleaning mechanisms, and sensor and controller units are reviewed in detail. The survey provides a reference for the readers to design and develop a window-cleaning robot for specific application.