Yield potential (YP) is defined as the "yield of a cultivar grown in environments to which it is adapted when nutrients and water are nonlimiting, and when pests, diseases, weeds, lodging, and other stresses are effectively controlled" (Evans 1987). Rajaram and Fischer (1989) described seven world wheat mega-environments (MEs). They are distinct niches of the developi ng world with an area that, by definition, is greater than 1 million hectares. After weighting wheat production within an ME according to per capita income, likely breeding progress and strength of the National Agricultural Research System, Fischer and Rajaram (1990) gave a priority assessment for CIMMYT wheat research. MEl (irrigated) had the highest percentage of developing world wheat production and it also had the highest priority for research. It has been found that varieties with high yield potential in MEl usually yield more in other MEs, even at moderate levels of stress. The past yield increases in MEl have paralleled increases in YP for this ME. CIMMYT has the responsibility of maintaining and increasing YP in a framework of a world mandate, therefore understanding and researching YP is a priority task. There are several aspects of YP that merit our attention: How is it determined? What morphological and physiological characters and in what combinations is expression maximized? What is the genetic variability in YP and traits related to it? How are these characters transmitted from one generation to the next? How can high yP genotypes be selected as early as possible in the improvement process and in an efficient and cost effective way? In what follows, we provide a physiological research framework geared to bread wheat YP improvement for MEl.