Premixing phase is the most important stage of the postulated severe accident scenario in the molten fuel–coolant interaction (FCI) of nuclear reactor. The premixing phase of FCI phenomena is characterized by the molten fuel material coming into contact with the liquid coolant. This usually occurs in the form of jets or drops of the molten material depending on the severity of the breach of the core. In the present chapter, the mechanism of fragmentation of jet into drops and further fragmentation of the drop into tiny particles is analysed in great details. Rapid and intense heat transfer from the molten drops leads to phase change of the liquid at high temperature due to film boiling. Understanding the underlying physics of the premixing phase comprising high temperature drops in coolant is important to mitigate the severe accident possibility. Experimental and numerical works on the premixing phase and break-up of jet and drops are reviewed thoroughly in the present chapter. © 2018, Springer Nature Singapore Pte Ltd.