Acquisition of energy is a prime objective of the search for nutrition. The energy budget of a population or trophic level comprises the sum of energy gains and losses by each individual organism. Since these energy exchanges are governed by the same thermodynamic principles that govern purely physical transfers, the animal energy budget can be developed according to the Thermodynamic Laws. The following points are discussed in this paper: (1) Energetics at the organismic or population level can be completely described by equations based on the algebraic statement of the First Law of Thermodynamics (ΔE = Q + W). (2) Care must be used in the definition of terms such as enthalpy, entropy, and free energy. The latter two, particularly, have been misunderstood and misused in the ecological literature. Analogies between bound heat (TΔS) and the respiratory heat loss (R) are misleading and logically unsound. (3) The possible advantage of using changes of free energy in ecological energetics is outweighed by the relative convenience and accuracy of methods for measuring changes in enthalpy. (4) There are theoretical errors in the use of respiratory gaseous exchange as an indirect estimate of metabolic heat loss by growing (as opposed to fasting) organisms.