When a couple with children divorces, French Family Law stipulates that the maintenance obligation of the non-custodial parent (the parent that does not live with the children) be supplemented with child maintenance payments to help in the caring for and education of the children. A Family Affairs judge fixes this amount during the divorce proceedings. In France, contrary to a number of foreign legislations, the judge fixes this amount without having recourse to an official guideline. The purpose of the first part of this article is to discuss the judicial and economic logic that may be advanced in order to justify the parameters of an official guideline that would satisfy the principles of equity, efficacy and incitation. The questions of evaluating the cost of a child, the period of reference for this evaluation, the rules for sharing between parents and eventual adjustments to the guideline are notably discussed, based upon economic reasoning and reference to foreign guidelines. In the second part of this article, nine guidelines are simulated from a real database (580 court decisions) in order to compare the amounts calculated according to these guidelines to the amounts actually decided upon by judges. The analysis of these results allows, on one hand, to see the impact of the various parameters of the guidelines according to several different family characteristics, and, on the other hand, to identify the guideline closest to that which the judges practice and is, therefore, the most likely to be accepted by the profession.