Electrolytes, as far as we athletes should be concerned, refer mainly to sodium (Na+) and potassium (K+). Na+ and K+ are very tightly regulated in the body. If an imbalance occurs, bad things happen. The most common and most dangerous electrolyte abnormality in endurance athletes is hyponatremia. In this article, I will discuss what hyponatremia is, what causes it, why should we care about it, and how can we avoid it.