Lescourret and Génard (2003) developed a multi-level theory of competition for resources applied to fruit production, considering that any collection of unit parts (cells or seeds in a fruit, fruits in an infructescence or in a tree...) can form a population and the population is subject to competition, whatever the level of organization. The principles of the theory are that the mass of each unit decreases when the number of units in the population increases and that the total mass of the population increases as the number of units increases until it reaches a maximum, after which it decreases. A three-parameter model based on that theory was used to analyse the level of density-dependence, i.e., the effect of the population size on its mass. Comparing the number of cells and the mean cell volume in mesocarp of fruits from peach genotypes showed a strong and undercompensating density-dependence that revealed competition between cells. Similarly, when comparing populations for different fruit species, similar results or exact compensation were observed for several levels (fruit, infructescence, tree…). Because undercompensating or exact density-dependence were found in most of the cases, a simpler two parameter model was proposed by Prudent et al. (2013). This two parameter model was used to analyse the genetic control of competition between seeds in multi-seeded fruits.