Covalent organic framework (COF) monolayers, with atomically thin, ordered networks, and rich functionality, are widely studied due to their unusual structure/property relationships. However, synthesizing COF monolayer has remained an unmet challenge due to the difficulty of controlling reactions at the monolayer limit with large-scale uniformity. The identification and development of new reactions and polymerization conditions are critical for the further advancement of COF monolayer materials. Moreover, as one-molecule-thick a freestanding films, COF monolayer offers an ideal material system. Many advanced applications of COF monolayer have been explored in recent literature. This review provides an overview of the current state of precise synthetic strategies for COF monolayer, highlighting the advantages and limitations of different synthetic approaches and key challenges related to enhancing quality, and emphasizing the unique benefits of COF monolayer as both an ideal model system and for advanced applications.