Methods to assess the size of a population and the interactions between populations in terms of biomass (weight of living material) or energy content are described. Biomass can be expressed as wet weight, dry weight (DW), shell-free dry weight (SFDW), ash-free dry weight, or as the amount of organic carbon present. The energy content of a material may be determined directly by oxidation, either by potassium dichromate in sulphuric acid, or by burning in oxygen and determining the amount of heat liberated. The latter method—bomb calorimetry—is most convenient and is widely used in ecology, but it involves drying the material, and volatile substances can be lost. Methods to estimate standing-crop, energy density, feeding and assimilation, and production are reviewed. Energy budgets can usefully be summarized and compared if the efficiencies of various processes are calculated. Dynamic energy budget models are introduced.