The evolution of the sunflower crop has been described by E.D. Putt. Cultivated sunflower has been the subject of a number of reviews that include agronomy, pests, diseases, morphology and physiology, and breeding and genetics. This chapter analyzes the contribution of inbreeding and heterosis to the improvement of sunflower yield and yield stability. The two major environmental stresses affecting worldwide sunflower production, namely, diseases and water stress, are analyzed separately. Sunflower rust not only affects interception of radiation and gas exchange, but also may reduce translocation of assimilate from leaves because of the sinklike activity of the growing rust colony, increased respiration of the diseased tissue, and reduced leaf assimilation. The chapter illustrates how simulation models could assist in the identification of traits important for yield improvement in specific environments. Breeding and selection has clearly improved sunflower yield potential.