Fluid therapy is a central tenet of both anaesthetic and intensive care practice, and has been a solid performer in the medical armamentarium for over 150 years. However, mounting evidence from both surgical and medical populations is starting to demonstrate that we may be doing more harm than good by infusing solutions of varying tonicity and pH into the arms of our patients. As anaesthetists we arguably monitor our patient's response to fluid-based interventions more closely than most, but in emergency departments and on intensive care units this monitoring me be unavailable or misleading. For this podcast Dr Paul Marik, Professor and Division Chief of Pulmonary Critical Care at Eastern Virginia Medical Center delivers a masterclass on the physiology of fluid optimisation, tells us which monitors to believe and importantly under which circumstances, and reviews some of the current literature and thinking on fluid responsiveness.