Crop yield in sugarcane is the result of how well the source leaves can supply the assimilates needed for the growth and filling of the harvestable plant. This is the source–sink relationship, which in sugarcane is still poorly understood. This chapter discusses the significant advances made in this area in the past few decades, as well as suggesting future trends in research such as high-throughput genomic, metabolomic and phenotypic analyses that could allow refinement in crop and metabolic modelling, as well as improved breeding and production strategies.