Determination of potassium level is one of the most frequent laboratory tests in clinical medicine. Hyperkalaemia is defined as a potassium level >5.0 mmol/L and is one of the most clinically important electrolyte abnormalities, because it may cause dangerous cardiac arrhythmia and sudden cardiac death. Here, we review methodological challenges in the determination of potassium levels, important clinical aspects of the potassium homoeostasis as well as of the pathophysiology of hyperkalaemia.