Fluid therapy in the patient with vomiting and diarrhea is essential to correct hypovolemia, dehydration, acid-base imbalance, and serum electrolyte abnormalities. Prediction of acid-base or electrolyte disturbances is difficult; therefore, point of care testing is beneficial to optimize therapy. This article focuses on the pathophysiology and treatment of hypovolemia, dehydration, electrolyte disturbances, and acid-base derangements resulting from and associated with vomiting and diarrhea.