Steady state simulation is now widely used for the design and optimisation of mineral processing plants. One of the most difficult unit operations to simulate in mineral processing is flotation. There are two main reasons for this. Firstly the flotation circuits are made up of a number of streams of widely differing flowrates and compositions, which can make the collection of reliable experimental data time-consuming and expensive. The data generally available are thus limited in number and often subject to experimental error Secondly, the flotation process is highly complex and controlled by many different hydrodynamic and physicochemical parameters. Many mathematical models exist which represent flotation, reflecting the variety of approaches with emphasis on different principal parameters, however; few of these models are effectively operational for industrial purposes, their usability depending on the data available. This paper presents the solutions to such problems provided by the USIM PAC 2 simulator Several methods are possible, depending on the objectives of the simulation study and the data available, that require the use of data reconciliation algorithms, simple flotation models (flowsheeting) or much more complex models (liberation modelling). A detailed description is given of the flotation models embedded in the software enabling different parameters for specific ores and specific plants to be taken into account. The more detailed the model is, the greater the amount of data required for its use. Industrial examples are given to illustrate some of the uses and relative advantages of the different models.