Yield is the most important breeding trait of crops. For fruit-bearing plants such as Solanum lycopersicum (tomato), fruit formation directly affects yield. The final fruit size depends on the number and volume of cell layers in the pericarp of the fruit, which is determined by the degree of cell division and expansion in the fertilized ovaries. Thus, fruit yield in tomato is predominantly determined by the efficiency of fruit set and the final cell number and size of the fruits. Through domestication, tomato fruit yield has been markedly increased as a result of mutations associated with fruit size and genetic studies have identified the genes that influence the cell cycle, carpel number and fruit set. Additionally, several lines of evidence have demonstrated that plant hormones control fruit set and size through the delicate regulation of genes that trigger physiological responses associated with fruit expansion. In this review, we introduce the key genes involved in tomato breeding and describe how they affect the physiological processes that contribute to tomato yield.