The distribution of biomass by size is an indicator of ecosystem structure. Single biomass size-distribution (BSD) studies are not likely to provide clear insights into the bioenergetic dynamics of an ecosystem since there is a high level of uncertainty associated with the estimation of energy and matter fluxes from only basic allometric equations and biomass size-distribution data. However, the use of body size (i.e. size-fractionation) as an organizing criterion to study energy fluxes in situ is very convenient both conceptually and methodologically. Since many processes related to energy fluxes at the individual level are size-dependent, the size-fractionation of a community permits the generation of compartments composed of organisms having approximately the same process rates. From the perspective of Hierarchy Theory, this characteristic of size-based compartments is a major advantage in comparison to other kinds of component aggregation (e.g. taxonomic). The simultaneous study of biomass size-distribution and in situ size-fractionated processes (e.g. respiration, primary production) should prove to be very useful from an empirical and modelling point of view.