A sound understanding of various aspects fluid, electrolytes and acid-base balance and its disturbances is critical in pediatric practice. Notably, water accounts for 70–80% of a neonate’s body weight and 55–60% of body weight by the age of 1-2 years. Total body water (TBW) = 0.61 × weight (kg) + 0.21. TBW is less in obesity since fat is known to have low water content. TBW consists of two major compartments, namely intracellular representing 30–40% of body weight and extracellular representing 20–25% of body weight, there are two minor compartments too: transcellular representing 2% of body weight and slowly exchangeable components representing 8–10% of body weight. Regulation of body water is controlled by its intake and excretion. Urinary excretion is controlled by two complementary mechanisms: ADH production, storage and release, and renal epithelial tubular cell response to ADH. Body water contains electrolytes with positive charge (cations) or negative charge (anions). Important cations are sodium, potassium, calcium and magnesium. Important anions are chloride, bicarbonate, sulfate, organic acids and protein acids. Sodium exists predominantly in the ECF whereas potassium and phosphates are primarily in ICF. ECF and CSF compartments are normally in osmotic equilibrium, except for transient changes. Disturbances of acid-base balance are in the form of metabolic acidosis/academia, metabolic alkalosis/alkalemia, respiratory acidosis/academia, and respiratory alkalosis. Alkalemia and mixed acid-base disturbances. For determining the acid0base status of a child a gadget, blood gas analyzer, is employed. It measures pH, PCO2 and Hb concentration. The remaining indices can be calculated from them only. As a rule fluid and electrolyte therapy consists of deficit therapy, maintenance therapy and replacement of ongoing losses. It is important to bear in mind the requirements in special situations such as diarrheal dehydration, malnutrition, acute renal failure, congestive cardiac failure and intestinal obstructions. For instance, in order to provide less sodium but more potassium in dehydration accompanying malnutrition, a special rehydration solution for malnourished children (ReSoMal) is recommended by the WHO.