This review article series on water and electrolyte disorders is based on the 'Electrolyte Winter Seminar' held annually for young nephrologists in Japan. The seminar includes lively discussions based on cases, which are also partly included in this series as self-assessment questions. The first article in this series focuses on pathophysiology, symptoms, outcomes, and evaluation of hyponatremia, a common water and electrolyte disorder in clinical practice. Diagnosing the root cause(s) of hyponatremia can be challenging due to various etiologies and co-morbidities that affect water and electrolyte homeostasis, which can result in inappropriate management and worse outcomes in acute and chronic hyponatremia. This review provides an overview of pathophysiology, symptoms, outcomes, and evaluation of hyponatremia for better comprehension and improved clinical practice.