In an era where health care expenditure control is a necessity, weighing a new treatment's cost against its added benefits is of crucial importance. These medico-economic analyses may be planned for the evaluation of new molecules in their daily use. Different methods may be used. The medico-economic evaluation of a drug can be performed with the use of clinical studies or modelization techniques. "Clinical studies" include the following methods: randomized trials, so-called "experimental", non-randomized studies, and observational studies. This classification follows a downward evolution of both the investigator's intervention power and the level of proof. Finally, modelization is becoming a commonly used tool. Randomized trials are the "gold standard" but the other, more pragmatic study types, providing a lesser degree of proof can complete the evaluation of a new molecule. One must find the best compromise between the study's objectives and the study type.