Charging of airborne particle is an important process in both scientific studies and industrial applications, of which the unipolar charging takes up the major part due to higher charging efficiency. In order to understand the charging mechanism of airborne particles, accordingly to predict the efficiency of unipolar chargers, different theoretical models have been proposed and investigated in all regimes. Numerical techniques have been also adopted by researchers to analyze the charging process of airborne particles and thus to model the unipolar chargers. On the other hand, with the advent and application of various measuring techniques, the electric charges measurement of airborne particles was made possible for researchers. In this work, we mainly present a summary of these models, numerical techniques and experimental results on unipolar charging in all regimes, as well as the recent developments in the design of unipolar chargers, trying to briefly overview the development in this research field.