Sugarcane is the primary raw material for sugar, energy, ethanol and other by-products. Although the yield of sugarcane per hectare has been increasing, there has been a plateau in the genetic gains in sucrose yield (kilograms of sucrose per tonne of milled cane). This has led to an increase in sucrose yield per hectare at the cost of higher harvest and processing expenses, impacting the profitability of this agroindustry sector. There is a trade-off between fibre and sucrose production, which limits the increase in sucrose accumulation without affecting fibre and vice versa. Therefore, the main challenge in sugarcane research is to enhance sucrose yield without compromising biomass production. Fibre and sucrose are produced by establishing a source–sink relationship, which involves the interaction of organs responsible for synthesising, transporting and storing carbohydrates. Modern breeding efforts aim to improve processes such as photosynthesis, the transportation, consumption and accumulation of photoassimilates. However, it is not possible to improve these processes without enhancing the complex signalling pathways and feedback regulation that involve both intrinsic and extrinsic factors. Intrinsic factors include regulation by end-products (sucrose) and hormonal signals, such as gibberellins, auxins, cytokinins, ethylene and abscisic acid. Gibberellins and auxins participate in the formation of the sink organ, which is the internode in sugarcane. These phytohormones trigger signalling pathways associated with cell division and elongation and the generation of sink strength through the induction of activity of enzymes, such as invertases and sucrose synthase. However, phytohormones, such as ethylene and abscisic acid, have a synergistic effect and provide major activity during the ripening stage through action on signalling pathways of defoliation, phloem loading and unloading, carbohydrate reallocation and the enhancement of activity by sucrose synthesis enzymes, such as sucrose phosphate synthase. This chapter discusses the source–sink relationship in sugarcane and its close relatives, including source strength and sugar movement, sink organ formation and sucrose utilization and accumulation. It critically analyses the entire source–sink relationship, providing an in-depth review of the main bottlenecks associated with phytohormone signalling. Understanding these signalling pathways will be useful in comprehending the dynamics of sucrose accumulation, enabling the identification of critical points that could be targeted in plant breeding to maintain efficient coordination of the entire source–sink relationship to improve sucrose yield without affecting biomass production.