The influences of hydrogen impurities on the performances of indium-gallium-zinc oxide (IGZO) thin film transistors (TFT) are summarized in this article. Firstly, the sources of hydrogen impurities in the IGZO channels of the TFTs are proposed, which could originate from the residual gas in the deposition chamber, the molecules absorbed on the sputtering target surface, the neighbor films that contain abundant hydrogen elements, doping during annealing processes, etc. The hydrogen impurities in the IGZO films can exist in the forms of hydroxyl groups and metal hydride bonds, respectively. The former originates from the reaction between H atoms and the O2- ions. This reaction releases free electrons, leading to a rise of the Fermi level of IGZO, and thus enhancing the mobilities of IGZO TFTs. The latter incurs negative charges on H atoms, and thus changing the distribution of the subgap density of states, hence improving the negative bias (or illumination) stabilities of IGZO TFTs. Subsequently, various methods are also proposed to characterize hydrogen elements in IGZO, such as secondary ion mass spectroscopy, thermal desorption spectroscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. Finally, the effects of hydrogen impurities on the electrical characteristics of the IGZO TFTs, such as the field effect mobilities, subthreshold swings, threshold voltages, on/off current ratios as well as the positive and negative bias stress stabilities, are discussed. The results indicate that hydrogen element concentration and process temperature are two key factors for the device performances. With the increase of hydrogen element concentration in the IGZO channels, the TFTs exhibit higher electron mobilities, lower subthreshold swings and better reliabilities. However, annealing at too high or low temperatures cannot improve the device performance, and the most effective annealing temperature is 200-300℃. It is anticipated that this review could be helpful to the IGZO TFT researchers in improving the device performances and understanding the underlying mechanism.