An understanding of the main factors influencing grain yield in soybean can provide key insights for making management decisions to increase yield. Seed number is determined by the amount of photosynthate produced between R1 and R5 that is allocated to the seeds, divided by the minimum amount of photosynthate needed to keep a single seed from aborting. Stresses or improvements in crop growth prior to flowering should not have a significant impact on final yield, provided that >95% light interception is achieved by R1. Seed weight is determined by the seed growth rate and the length of the seed fill period. Simplified, soybean yield is mainly determined by photosynthate production from R1 to R5 and the length of the seed fill period. Management practices should focus on maximizing photosynthate production during seed set to increase seed number and limiting stresses during seed fill to extend the seed fill duration and increase seed weight.