Keywords: cell division, cell growth, cell endoreduplication, fruit growth, genotype, G×E interaction, model, tomato. Fruit size is a major component of fruit yield and quality of many crops. Variations in fruit size can be tremendous due to genotypic and environmental factors. The mechanisms by which genotype and environment interact to determine fruit size are complex and poorly understood. Genotype-by-environment interactions emerge from cellular and molecular processes underlying fruit growth. In this thesis the basis for variations in tomato fruit size was analysed through the development of a dynamic fruit growth model integrating three fundamental fruit cellular processes: cell division, cell growth and cell endoreduplication. Experiments were carried out to understand the link between cellular processes and fruit growth and their responses to genotypic factors, contrasting fruit loads and temperature conditions. Experimental data showed that the contribution of cell number and cell size to the genotypic variation in final fruit size depends on the timing of assimilate supply to the fruit. Genotypic variation in fruit fresh weight, pericarp volume and cell volume was linked to pericarp glucose and fructose content. Genotypic variation in cell number was positively correlated with variation in pericarp fructose content. Reduction in final fruit size of early-heated fruit was mainly associated with reduced final cell volume in the pericarp. Early heating increased the number of cell layers in the pericarp, but did not affect the total number of pericarp cells significantly. Continuously heating of a fruit reduced anticlinal (direction perpendicular to fruit skin) cell expansion more than periclinal (direction parallel to fruit skin) cell expansion. Information derived from the experiments was incorporated into a dynamic model of fruit growth. The model describes fruit growth from anthesis until maturation and covers the stages of cell division, endoreduplication and cell growth. Model development relied on understanding and integrating biological interactions between processes at the cell, tissue and fruit scales. The model was parameterized and calibrated for low fruit load conditions and was validated for high fruit load and various temperature conditions. The model was able to accurately predict final cell number, cell mass and pericarp mass under contrasting fruit load and most of the temperature conditions. Model sensitivity analysis showed that variations in final fruit size are mainly associated with variations in parameters involved in the dynamics of cell division. Among these parameters, cell division duration had the strongest influence on final cell number and pericarp mass. The model can be used to carry out virtual experiments with treatments that are difficult or impossible to test experimentally and allows for predicting and analysing fruit growth responses to genotype-by-environment interactions. This thesis has contributed to closing the gap between genotype and phenotype related to tomato fruit growth. An integral and coherent development of models at relevant levels of plant organization can further help to close this gap.