This book undertakes an analysis regarding the taxation of Circulation of Goods and Services Tax (ICMS) levied on Electronic Commerce (e-commerce) by the end consumer, who is not the taxpayer, through interstate operation. Therefore, it is a comparative study of the rules and incidence of this tax in national and international level as well as the conflicts generated by the federal entities, conforming one type of "fiscal war", due to the application of the "origin principle", which no longer meets the Federation goals. The analysis addresses the importance of this new form of commerce, from the description of the evolution of e-commerce, as well as the lineation of advances and prospects for this type of trade. It is given a special focus on the economic impact due to the current model of collection of tax on consumers States, notably the state of Piauí, showing its influence on deepening regional inequalities. What was found was that the process of globalization including technological innovation and the resulting modernization of the economy, imposed new challenges and an urgent need for constitutional and legal adaptation of the National Tax System. This is evident in the decision of the Supreme Court regarding the basis used when the declaration of unconstitutionality of the 21/2011 ICMS Protocol which deals with the matter, though, some of them deserve a deep reflection. Maintaining this factual situation favors the fiscal imbalance of the states and commits the federative pact due to a possible violation of constitutional principles. The new Constitutional Amendment (EC 87/2015) may have been the first step in a necessary transition from the rule of origin to the rule of destination, advocated by this thesis.