Cooperative interstate relations are essential for the maintenance of the economic union and the political union established by a confederacy or a federacy. This suggests that interstate relations would be featured prominently in the literature of the U.S. federal system, yet relatively few scholars have studied horizontal state relations. This volume provides detailed information and an analysis of interstate relations, and advances recommendations to improve the economic and political union. The ultimate goal is to stimulate scholarly research on important yet neglected interstate issues.