Article is dedicated to examining the most important issues related to the legal invalidity of juristic acts. The article deals mostly with modern Russian Law and the author shows also short background problems of nulls, void and non-existent acts. Then the author analyses the institution of invalidity in Russian Law, types of invalid acts according to the modern Russian Civil Code. In this article an attempt has been made to look for the proper criteria for the differentiation of absolutely invalid acts of law and “actionable” acts according to Russian substantive Civil Law. The text shows the classification of invalid acts of law in the Russian Civil Code and their consequences. The main purpose of the article is to reflect on the questions of how to understand invalidity as a legal construction and how it relates to the “unfinished” acts of law under Russian Law which could be understood as practically non-existent acts of law.