This chapter provides a detailed analysis of diuretic pharmacology, emphasizing their clinical applications in anaesthesia and critical care medicine. The chapter begins by reviewing the physiological mechanisms governing renal function and fluid balance, setting the context for understanding diuretic actions. It systematically examines key diuretic classes, including loop diuretics, thiazides, potassium-sparing diuretics, carbonic anhydrase inhibitors, and osmotic diuretics. Each class is explored thoroughly, covering their specific mechanisms of action, pharmacokinetics, therapeutic indications, dosing strategies, and potential adverse effects. Particular emphasis is placed on loop diuretics due to their central role in managing perioperative fluid overload, pulmonary oedema, and acute kidney injury. Additionally, the chapter highlights important drug interactions and electrolyte disturbances associated with diuretic therapy, offering practical guidelines for their safe and effective use in clinical practice. By combining foundational concepts with clinical applications, this chapter equips trainees with essential knowledge to optimize fluid management, enhance patient safety, and ensure effective treatment in perioperative and critical care settings.