The quenching and partitioning (Q&P) process is a novel heat treatment for the development of advanced high strength steels that is raising an elevated interest by steel makers and steel researchers around the world. The reason is that reported results on mechanical properties, showing promising levels of forming and strength, are proving this new type of steel as a serious competitor of TRIP, DP and martensitic steels. The Q&P heat treatment consists of an initial partial or full austenitisation, followed by a quench to form a controlled amount of martensite and an isothermal treatment to partition the carbon from the martensite to the austenite. Although the path of the heat treatment is simple, the investigations have shown that the evolution of the microstructure during the application of the Q&P process is rather complicated. Processes occurring during the partitioning step, such as the migration of the interfaces, the carbon accumulation near the austenite interfaces and the carbon diffusion through ferrite, have strong effects on the resulting microstructure. In this work, the most important microstructural changes found during the application and simulation of the partitioning step of the Q&P process are analysed and discussed. Procedures to control the microstructure development in the application of the Q&P process are proposed.