Critically ill hypotensive patients can be difficult to assess. The first question to be addressed is whether the patient is likely to be preload responsive and the second is whether the patient actually needs fluids. Predicting fluid responsiveness does not always imply the need for cardiac output augmentation, since normal subjects are normally fluid responsive. The presence of preload responsiveness parameters should trigger fluid administration if there is evidence of tissue hypoperfusion. Excess fluid which is not required or to which the patient is not responsive can have definite deleterious effects. Functional hemodynamic parameters are based on cardiopulmonary interactions. ‘Filling pressures’ per se are of no value. The profusion of more sophisticated techniques indicates that no technique is perfect, and all are subject to limitations and variation in accuracy in different clinical conditions. Monitoring by itself does not change outcomes, and the demonstration of preload and preload responsiveness parameters in clinical practice needs to be incorporated and evaluated in a consistent management protocol to improve the patient outcome in diverse clinical situations and environments. Currently the evidence supports the use of functional dynamic preload responsiveness over static preload parameters to distinguish fluid responders from non-responders, but technical aspects, limitations, availability, and suitability for different clinical conditions must be considered. The authors use predominantly echocardiographic techniques but there is no single parameter that can be useful under all clinical conditions.