Although exports of underdeveloped countries (UDCs) are currently for the most part primary products, with manufactured exports accounting for only about 15 per cent of the total, the future growth of these exports is crucial to the success of efforts to achieve economic development in these countries. This proposition holds regardless of whether one accepts the widely held view that the outlook for primary product exports is extremely bleak. That view may turn out to be too pessimistic. The proposition that there is a secular tendency for the prices of primary products to decline relatively to those of manufactured products is not well supported by either the empirical evidence (unless that evidence is presented rather selectively) or by theoretical argument. The claim that prices have a long-run tendency to move against primary products and in favour of manufactured products is often the premise to an argument that the UDCs will have to move out of specialization in primary products towards the production and export of manufactured products. But the conclusion does not depend upon this premise. Production of primary products will not be a sufficient basis for economic development even if the trends in primary product prices turn out to be extraordinarily favourable. A few countries in specially favourable circumstances may be able to raise income per head to high levels by achieving high levels of productivity in the production of primary products and obtaining their requirements of manufactured products through international trade.