This chapter analyzes past breeding efforts, their affects on yield-related physiological traits, and the challenge of further raising the yield potential of the rice varieties. The principal emphasis in the chapter is on the physiological traits that are related to yield. Understanding the relationship of physiological traits and yield will assist intelligent use of physiological components in breeding for high yield. As regards physiological traits, the promotion of photosynthetic activity by fertilizer N should be as great as possible, and that of respiration as little as possible. In Korea, high-yielding varieties derived from indicajaponica hybridization had large sink capacity and more inner vascular bundles than japonica varieties. Therefore, rice yield can be increased by increasing total dry matter and/or harvest index. Studies on sink-source relationships in rice have led rice physiologists to believe that physical environments need not limit rice yield beyond the current levels.