Electrolyte disorders are very common in the pediatric population. Derangements in serum sodium and potassium concentrations are among the most frequently seen given the risk factors and comorbidities unique to children. Pediatricians, in both outpatient and inpatient settings, should be comfortable with the evaluation and initial treatment of disturbances in these electrolyte concentrations. However, to evaluate and treat a child with abnormal serum concentrations of sodium or potassium, it is critical to understand the regulatory physiology that governs osmotic homeostasis and potassium regulation in the body. Comprehension of these basic physiologic processes will allow the provider to uncover the underlying pathology of these electrolyte disturbances and devise an appropriate and safe treatment plan.