The 2 1 st century challenges for genetic improvement are as they have always been and as they will always be for any century: lkeep it simple; 2harness the theory and the practice of selection into the shaping of an ideal that is economically meaningful within the context of the specific production and marketing environments; 3creatively and effectively combine the available stocks in the shaping of that ideal (i.e., efficient crossbreeding systems); 4the wise use of available technologies that could foster the accomplishment of that ideal. In the 20th century, the dairy, swine and poultry industries evolved into extremely powerful and efficient breeding machines and production systems. Some costs accompanied the success, and to contain these while maintaining the pace of progress will be one of their main challenges in the next century. In beef cattle breeding, remarkable accomplishments were made throughout the 20th century in the development of adapted types that can survive, reproduce regularly and grow in a multitude of production environments. For the 21st century, the genetic challenges will be to continue to focus on the overall biological and economic efficiency of the total production system, but including in that effort the consideration of those characteristics that determine consumer acceptability of the final product. For this to happen, the development of an economic alignment between the different segments of the industry is one of the most important challenges to be met. The 20th century was notable for a genetic concern about means, while in the 2 1 st century, genetic improvement must accommodate concerns regarding both means and variances. The U.S. sheep industry is in a period of crisis, and their first genetic challenge for the 21st century will have to be the definition of a clear and realistic vision for the role that the industry is to play in the broader context of agricultural and marketing systems. The essence of genetic improvement will remain the same, but change will be a constant, and to be prepared to anticipate and to cope with change will be one of the key challenges for the livestock industries in the 2 1 st century.