In this Meet-the-Professor session, I will cover the diagnostic and therapeutic aspects of hypotonic hyponatraemia that are relevant to clinical endocrine practice by using an interactive, case-based format. Illustrative clinical cases will be presented to formulate a systematic approach to determining the correct underlying cause(s) of hyponatraemia, which is essential to guide appropriate treatment. I will review the limited evidence regarding the benefits and risks of the available treatment strategies, and highlighted current controversies and evidence gaps.