Three distinctive schools present diverse interpretations of agricultural marketing. One of these derives from the economics of the farm business and casts marketing as all that happens to products past the farm. A second centres on marketing's co-ordinative role. A third is in allegiance to market development. Contemporary problems, beginning with those of the 1920 price reductions in the United States and accentuated by the worldwide depression of the 1930's, primarily explain the periods of heightened interest in marketing. In the early 1970's the common thread is institutional change in marketing, which originates in events as diverse as declining importance of assembly markets for price discovery, and the redesign of marketing within trading blocs such as the European Common Market.