Body size is one of the most fundamental organism characteristics because it is linked to variation in a huge range of physiological, morphological, ecological, and evolutionary traits. Body size is highly plastic and can evolve quickly, but the mechanisms enabling body size change and the fitness landscape for body size evolution are still unclear. I have postulated that a dynamic feedback loop links environmental signals set by body-size dependent processes back to the evolution of size itself. My research program focuses specifically on the reciprocal questions of how body size, energy use, and consumer-resource interactions generate ecological patterns and how energy supply and demand in turn drive body size evolution. The figure to the left illustrates this reciprocal focus on body size and environmental factors. My research falls into several categories described by the arrows in this figure.