This paper begins with a brief historical presentation of the evolution in France of evaluation practices and the laws and regulations that impose them. The French example is an illustration of a general development of evaluation methods and of the objects that these methods seek to grasp. The usefulness and implementation of ex post evaluations will then be discussed, whether by simple experience feedback or by methodological necessity, in particular for the analyze of wider economic benefits (WEB) which were often mentioned in the presentation of the projects and which were sometimes decisive in the decision to invest. This methodological dimension will naturally lead to discussing the use of permanent observatories, such as those set up in France after the construction of major motorway projects or new high-speed rail lines. Particular attention will be paid to the case of the socio-economic observatory set up to assess the effects of the “Sud Europe Atlantique” high-speed line, which is a first case of railway concession and which is a very singular project regarding its cost (€8 billion).