Major weapon systems and platforms, by nature, tend to be extremely complex and long-term purchases, incorporating cutting edge technology that is both developmentally complex and very expensive, often only being developed as the platform is built, put into service and updated, as has often been the case with new generations of fighter aircraft and warships. It is also the case with such systems that, like a hydroelectric dam or nuclear reactor, they cannot be acquired piecemeal but must be purchased and constructed all in one piece. This complexity, high costs and the lack of severability of contracts once signed are characteristics of procurement processes that generally trigger hard-fought political battles about costs, benefits and platform choices. The chapter lays out a four-fold categorization of procurement to address these complexities.