Body size is an attribute of individual organisms. It affects, or at least is correlated with, a considerable array of physical, physiological and behavioural characteristics that determine where individuals occur and what they do (Elton, 1927; Peters, 1983; Brown et al., 2004; Woodward, Speirs & Hildrew, 2005c). Such ubiquity makes body size an obvious candidate source of general rules that can be derived from selective processes acting on individuals, but which can also be applied at higher levels of ecological organization (Cousins, 1980; Peters, 1983; Dickie, Kerr & Boudreau, 1987; Yodzis & Innes, 1992; Cohen et al., 1993; Gaston & Blackburn, 2000; Cohen, Jonsson & Carpenter, 2003; Emmerson & Raffaelli, 2004). Although body size was identified as a key link between natural history aspects of population dynamics and community structure many decades ago (e.g. Hardy, 1924; Elton, 1927; Hutchinson, 1959), these early ideas have languished somewhat until relatively recently. In the last two to three decades a plethora of allometric body-size scaling relationships have been described and used to derive mechanisms to explain, or parameters to model, general patterns in natural systems (e.g. Peters, 1983). Recently, it has been suggested that basal metabolic rate is the fundamental constraint that underpins many of the size-related patterns and processes observed in natural systems, and that a metabolic theory of ecology could, eventually, attain similar importance to that of the genetic theory of evolutionary biology (Brown et al., 2004; Brown, Allen & Gillooly, this volume).