Fluid administration in the operating room is a cornerstone of perioperative hemodynamic optimization Functional assessment of fluid response aims at evaluating the hemodynamic changes associated with the interplay between heart and lungs on flow and pressure parameters, with the purpose of tailoring fluid administration to predefined physiological targets and specific patient needs. In patients under mechanical ventilation, the fixed and repetitive inspiratory and expiratory pressure changes affect right ventricle’s preload, afterload and, hence, stroke volume, finally determining the changes on the dynamic indices of fluid responsiveness, such as pulse pressure variation and stroke volume variation. These changes may even be enhanced by the application of maneuvers that may potentiate heart–lung interactions, the so-called functional hemodynamic tests. This chapter analyzes methods and variables to assess fluid responsiveness in the perioperative setting, how to perform functional hemodynamic tests and how to interpret them considering potential confounding factors and limitations.