Hyponatraemia is common and often a source of confusion for junior doctors. It is infrequently dangerous, but when it is, is a medical emergency and requires urgent treatment to avoid life-threatening cerebral oedema. Treatment of acute hyponatraemia is also potentially hazardous; it is therefore important to be able to recognise when urgent management is not indicated, and to investigate appropriately. This paper focuses on these issues, which are most likely to be the cause of consternation for the junior doctor. Recommendations are largely based on the 2014 joint European clinical practice guidance for management of hyponatraemia; the 2010 GAIN (N Ireland) guidance and 2013 American guidance are also referenced.