Publisher Summary This chapter examines the mineral requirements of finfish and compares the mineral metabolism of fish with that of other animals. Fifteen trace elements are considered to be essential in animals. Among these, the physiological role of a deficiency of chromium, cobalt, copper, fluorine, iodine, iron, manganese, molybdenum, selenium, and zinc is well recognized. Most of these trace elements have been detected in fish tissues; however, the essentiality of only a few of these elements has been demonstrated. The main functions of essential elements in the body include the formation of skeletal structure, maintenance of colloidal systems (osmotic pressure, viscosity, diffusion), and regulation of acid–base equilibrium. They are important components of hormones, enzymes, and activators of enzymes. Calcium and phosphorus are required for the formation of the skeletal structures of the body. Sodium, potassium, and chloride, along with phosphates and bicarbonates, maintain homeostasis and the acid–base balance. A fixed number of specific trace metals are firmly associated with a specific protein in metalloenzymes, which produce a unique catalytic function. Although aquatic organisms readily absorb metals from the environment, their ability to regulate abnormal concentrations varies with the species. Defining the role of inorganic elements in immune response, disease prevention, and reproduction of fish is an important and interesting area of research. Recent advances in molecular biology, microscopy, and analytical chemistry now provide us with many opportunities to explore the trace element metabolism and regulation in fish.