Potassium is a main cation in the intracellular compartment, with nearly 80% of stores being present in muscles, liver, bone and red blood cells. Intracellular potassium concentration is about 140-150 mmol/l in contrast to more than 30 times lower extracellular levels (3.5-5.0 mmol/l).This transmembrane gradient is crucial for maintenance of the membrane resting potential and neuro-muscularfunctioning, including the heart. In this review the physiological regulation of potassium balance is shortly reviewed to set a background for understanding its disorders.