[Truncated abstract] In ecomorphological or ecophysiological studies, variation in `design? traits (e.g. size, morphology and physiology) is thought to determine variation in ecologically-relevant performance traits, which in turn determines fitness in a particular habitat (Arnold 1983). Thus, natural selection is thought to act most directly on intermediate traits such as measures of locomotory performance. This thesis examined this process in the closely related group of Australian varanids lizards (Squamata: Varanidae). Phylogenetically, varanids are divided into three major clades. Size (mass and snout-to-vent length) is strongly correlated with these three clades. Two clades, (Gouldii and Komodoensis) are large, while the third clade (Odatria) has a smaller body size. Thus, there is considerable variation in size for various species. Size varied for species by three orders of magnitude. Size is also related to two ecological characteristics, foraging mode and habitat openness. Widely-foraging species were larger than sit-and-wait strategists, while species from open habitats were larger than species from semi-open or closed habitats. However, given the tight link between size and phylogeny we cannot separate adaptation of size to ecological traits from that of phylogenetic patterns. Of interest throughout this thesis was how variations in design (e.g. morphology and physiology) were related to ecological characteristics. Since body size also influences many of these morphological and physiological characteristics it is often necessary to remove the effects of size. Three design traits were examined in detail: body dimensions, vertebral number and metabolic rates. …Curiously, no performance variable linked differences in size-free body dimensions to retreat sites. This suggests that there is either a direct link between design and ecology (e.g. dorso-ventral compression), or some unmeasured performance variable related to retreat site. Given that most performance traits are thought to involve kinematic movements of the hindlimb limb and pelvis, and these were not best related to retreat site, then a direct link between design and ecology with respect to retreat site seems possible. In summary, this thesis provides evidence not only for links between design and ecology mediated by locomotory performance traits, but also direct links between design and ecology, for Australian varanid lizards.