||In the 1988 draft of CIMMYT's strategic plan CIMMYT in the Year 2000 and the document Report of the Third External Programme Review of the International Maize and Wheat Improvement Centre of the same year, wide adaptability (also termed "broad adaptation") to conditions over a range of broad agroecological zones known as "mega-environments" was recognized as one of the major thrusts for future wheat breeding efforts in an international context. The authors of the present article share the viewpoint that centers like CIMMYT should invest their resources in broad adaptability, leaving the task of breeding for specific adaptation to national agricultural research programs. CIMMYT's wheat breeding program dates back to 1944 as a bilateral venture between the Rockefeller Foundation and The Mexican Ministry of Agriculture. This program received an international mandate in the 1960's and continued evolving into the truly global operation it is today. Critics of CIMMYT's wheat breeding program have labeled the concept of wide adaptation as a banner for the propagation of a single, global variety. However, a careful examination of the facts will expose the flaws in such reasoning. Past products of breeding for wide adaptation at CIMMYT contributed to the Green Revolution, and our 45 years of existence on the global scene attests to the strong viability of wide adaptation as an alternative breeding methodology. On the other hand, while it is our strong belief that future work should exploit the strength of past plant breeding principles and methodologies, CIMMYT's wheat breeding program should also consider better alternatives if and when these become available. Indeed, over the last 45 years the concept itself of breeding for broad adaptation has greatly evolved, as new and distinct geographic areas came under the Wheat Program mandate. At present, the Program seeks to develop materials with general suitability for the five basic mega environments listed below (the year the Program began work specifically related to each mega-environment is given in parentheses): 1. Irrigated, optimum input ( 1944) 2. High rainfall (optimum for moisture but distinct from mega-environment I in the disease spectrum and the epidemiological vulnerability to various pathogens, including Septoria tritici 1972) 3. Semiarid and characterized by drought stress ( 1975) 4. Acid soil, especially aluminum toxicity (1975) 5. Wanner and characterized by heat stress (1981). The results of ISWYN since its inception have guided CIMMYT breeders in a dynamic program based on carefully conducted shuttle breeding. Over the years, these trials have revealed important genotype x environment interactions and allowed us to identify the five mega-environments proposed for spring wheats. So far, this information has proved relevant only for preliminary analyses done on ISWYN data; this may change, however, once the reliability of actual climatic data and genotype x environment interaction is recognized. Such a change would highlight the importance of the ISWYN. This report provides global, mega-environment, regional, and local summaries on data for the yield, agronomic traits, and disease response of 23rd ISWYN entries.