Multi-frequency bioelectrical impedance analysis (MFBIA) has been, and likely will increasingly be, used to rapidly and non-invasively assess the time course of volume losses and recovery in horses. Dehydration in performance horses is frequently the cause of health and performance problems, and presently used techniques for objectively quantifying optimum hydration are time consuming and challenging to perform accurately. Dehydration can take a number of different forms, with a balanced loss of water and electrolytes from both extra- and intracellular fluid compartments, or a primarily extracellular or intracellular dehydration. This review summarises the current state of knowledge regarding the quantification of dehydration, losses of water and electrolytes from extra- and intracellular fluid compartments. The effects of dehydration on exercise performance, muscle function, cardiovascular function, thermoregulation and feeding are briefly summarised. The review provides a quantitative description of the magnitude and time course of compartmental fluid losses and recovery in horses in response to feeding and due to exercise at different intensities and durations representing the endurance horse to the track race horse. Effective rehydration requires knowledge of the losses from the main body fluid compartments, which is now possible using MFBIA technology. The present review outlines the key approaches that have been used to assess dehydration in horses, including the new technique of MFBIA.