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Water and nutrient availability limit plant growth in a11 but a very few natural ecosystems. They limit yield in most agricultural ecosystems, and in the United States and other industrialized nations, intensive irrigation and fertilization have generated serious environmental problems. The acquisition of soil resources by plant root systems is therefore a subject of considerable interest in agriculture and ecology, as well as a complex and challenging problem in basic plant biology. Symbioses between roots and otlier organisms (notably mycorrhizas and N-fixing bacteria), modification of the rhizosphere through root exudates, and the uptake and transport characteristics of root axes are a11 important dimensions of this problem that are being actively researched by plant biologists. Another aspect of this problem that has received less attention, despite its probable importance, is root architecture. Recent methodological innovations present opportunities for improved under.standing of the functional importance of root architecture in the efficient acquisition of soil resources and plant adaptation to suboptimal soil conditions. The purpose of this Update is to briefly summarize conceptual issues and recent developments in the study of root architecture and to propose a framework for understanding its physiological basis.