The hydro-electrolyte and acid-base balance is of vital importance to maintain correct homeostasis, as it regulates most of the organic functions. It is a complex subject in its therapeutic management due to the varied and not easy formulas designed for the correction of these alterations. The aim of this work is to review the subject in an updated way, showing a novel method that can facilitate the correction of dehydration, but for the first time taking into account the basic daily fluid needs, which have never been taken into account by the previous methods. Two sensitive and necessary chapters are also presented, both for the surgeon and for the specialists who also have to deal with surgical cases. We are referring to the management of fluids during surgery, as well as in patients with chronic renal insufficiency, in which in most cases it is not known with certainty what strategy to follow in the administration of fluids. Acid-base alterations are not included in this work,as they do not form part of the objective of this review.