La nature juridique de l'impôt dans l'ancienne et la nouvelle économie du droit fiscal.

After specifying the legal nature of tax law as a coercive power application and as a public authority expression, this article examines the way economic tax theories acknowledge these essential fiscal dimensions. Indeed, tax constraint induces a behaviour modification, so that the legal rules produce important economic implications. We first consider the particular position of the old French public financial economics ; then, we underline some public economics' failures, and especially the optimal taxation theory ones. These flaws relate to the economic, fiscal and institutional specifications of the models. We present, then, the new fiscal economics of law framework of analysis. It pretends to take into account tax law and institutions' complexity. Finally, we draw a first critical assessment and propose a future research program, argued in three main directions. In addition to the fact that thenew fiscal economics of law carries out an analysis based primarily on costs, it under estimates the tax evasion results in terms of equity. The suggested framework ignores the way legal conflicts between the taxpayer and the administration are really resolved. This leads us to analyze the institutions'evolution issue. Thus, a true methodological renewal is needed.Tax law, efficiency costs, economics of law.