Chilldown process is a complicated process involves unsteady two-phase heat and mass transfer and initiates the cryogenic fluids transport. To advance the understanding of this process, both experimental and modeling investigations have been carried. The chilldown process can be generally divided into three regions which are associated with different flow regimes and heat transfer mechanisms. Under low flow conditions, the two-phase flow during chilldown is dispersed. The statistic feature of the liquid filaments is studied and a model, in which the heat transfer at the bottom is considered as a sum of vapor and liquid components, has been developed for the dispersed flow film boiling. The bottom wall heat flux is found to decrease under microgravity condition. A cryogenic chilldown model has also been developed. The model focuses on both vertical tube orientation and microgravity chilldown and shows a good agreement with previous experimental data.