When plantwide optimization problems must be decomposed and solved in pieces due to numerical complexity or practical concerns, if is natural to view each plans division as a separate profit maximizer. For a division to maximize ifs profit, however it must know the prices of interprocess streams. It is claimed that the correct way of finding these prices is to define slack resources over the process streams and price them using slack auctions. When slack auctions terminate properly, it can be shown that the plant divisions＇ solutions are globally optimal. How slack auctions are related to Lagrangean relaxation and how they can overcome some of the problems that standard decomposition and relaxation techniques have are discussed.