Microsimulation is a quantitative method for estimating the expected impact of an intervention (e.g. the modification of a tax rate) and describing its effects (winners, losers, budgetary envelope, effect on inequality indicators). It is based on taking into account the characteristics of the target population (e.g. age, income, etc.) and modelling public policy effects concerning this population. Because of the diversity of situations that it makes it possible to integrate, this technique provides more precise and complete results than estimates based on average or aggregate reasoning of the representative individual type. Its development has been encouraged by the improvement in computing power and the increase in statistical information (surveys or administrative data). It is an essential tool for ex ante evaluation of the impact of public policies and can also be used for ex post evaluation.