Fluid and electrolyte therapies including nutritional support are markedly developing in medicine and many kinds of commercial fluids are being introduced to clinical practice. Understanding the characteristics and usefulness of intravenous fluids is necessary to manage patients properly. Disputes about the usefulness of crystalloid or colloid solutions for specific clinical conditions still continue. To make ideal fluid therapy possible, many kinds of fluids will be developed and applied to clinical practice in the near future by mimicking the composition and functions of human body fluids. The selection of crystalloid and colloid solutions for fluid therapy should be performed by considering patients' clinical and pathophysiological conditions and the characteristics and usefulness of each intravenous fluid.