Excavation of plants from soil readily reveals that the shape of root systems is quite complex. The elegant drawings of Kutschera (e.g., Fig. 7.1; Kutschera and Lichtenegger 1992), collected painstakingly over many years of fieldwork, provide beautiful illustrations of the complexity and diversity of the spatial configuration of root systems, or ‘root architecture’. The possible importance of this complexity and diversity has been considered in the scientific literature for over a century (e.g., Anonymous 1887; Cannon 1911; Weaver 1926). Speculation has primarily focused on the role of root architecture in the acquisition of water (e.g., Pages et al. 1997) and nutrients (reviewed below), and in support of the shoot (Coutts 1983; Ennos and Fitter 1992; Bailey et al. 2002), although other important functions include storage, signaling processes (Aiken and Smucker 1996), disease and insect resistance, and belowground symbioses (e.g., Hetrick 1991). Although the focus of this chapter is the role of root architecture in nutrient acquisition, root architecture can best be understood as an aggregate of traits that co-optimize plant adaptation to several current and potential constraints. By analogy, the functional importance of shoot architecture has been fruitfully analyzed as co-optimizing solutions to the needs for structural support, light capture, and propagule dispersal (Niklas 1997). For roots, architectural patterns may co-optimize the acquisition of immobile soil resources such as phosphorus, the acquisition of mobile soil resources such as water, and support of the shoot. It must also be considered that roots exist in a hostile environment in which any given root may be lost through abiotic or biotic stress, so redundancy may be a virtue in root architecture. The term ‘root architecture’ is used to refer to the spatial configuration of a root system, excluding anatomical or morphological features of individual roots such as root hairs, but including assemblages of connected roots or entire root systems (Fitter 1991a; Lynch 1995). Root architecture is dynamic and environmentally plastic. It results from an aggregate of specific physio-