Large-scale synthesis of monodisperse nanoparticles is highly desirable for practical applications of nanoparticles in various fields of emerging technology. Among colloidal synthetic routes of monodisperse nanoparticles, heat-up process, which involves a gradual heating of precursor solution in a batch reactor, has received utmost interest after its successful size-controlled synthesis of various kinds of nanoparticles. In this essay, we discuss the fundamental research regarding the synthesis of monodisperse nanoparticles and describe how researchers developed the heat-up process. The mechanistic insights into this synthetic method and the discovery of size-dependent properties of monodisperse nanoparticles are also presented. We further emphasize its great impacts on utilizing monodisperse nanoparticles synthesized via the heat-up process for biomedical technology, energy conversion and storage devices, as well as electronic and optoelectronic devices.