The manipulation of sink to source relations has been subject to extensive plant breeding programs aiming to improve harvest index and thereby crop yield. The introduction of molecular and biochemical tools has enabled scientists to investigate the underlying principles. This has opened up the fascinating possibility of identifying molecular determinants of sink strength and to further increase yield on a rational basis. In the past, transgenic plants with alterations in the activity of only one putative molecular determinant have been created and this strategy has not resulted in substantial and reliable increases in yield. Yet, careful molecular and biochemical investigations have provided valuable insight about carbon flux into different metabolic pathways at different stages of sink development and it has become apparent that this metabolic channelling needs to be exploited by using stage- and cell-specific promoters in attempts to increase sink strength.