A new approach to understanding the classical conceptions of the public finances theory is closely connected with looking for ways out of the present world financial crisis. Scientific ideas of the outstanding German scientists A. Wagner (1835-1917) are of prior importance in this process. The Wagner’s law and his conception of fiscal economy can well become a means of knowing the current finance and economic processes. This article focuses on the nature of fiscal economy and its characteristic features. Wagner’s original view of the tax principles and taxation conditions is a constituent. part of his fiscal economy conception. His discovering not only a fiscal but also a social function of taxation was a breakthrough in this field at his time and remains topical at present. To Wagner fairness levying taxes was of great importance in order to understand the principles of efficient taxation. After analyzing the methods and theoretical foundations of the formulation of the Wagner’s low we were able to discover the causes and essence of the law. This article besides the formulation of the Wagner’s law, deals with the experience of testing its first and second version on various statistics materials of the different countries. Investigating the peculiar ways of reasoning and formulating the Wagner’s law, defining the approaches to test it on the empirical material gives an opportunity for further scientific research of the current problems of the public finances.