This chapter presents current knowledge, methods, and applications of nutritional energetics in an aquaculture setting. All organisms ingest and metabolize macronutrients that supply dietary energy and the nutrients have different fates. The different aspects of energy flow from feed to fish or shrimp are similar to those known in land animals or birds, but the aquatic mode of life imposes challenges and confers advantages both at the cellular level and at the whole organism level. Energy gain and losses are governed by the efficiency with which the macronutrients are utilized. The application of net energy system as prevailing in monogastric land animal nutrition is also valid for aquatic organisms. The large number of species reared by aquaculture is a real challenge, but many of the basic principles are valid across species. There is a need for precisely quantifying the partition of dietary digestible energy for somatic growth, reproduction, and other physiological changes due to environmental factors.