The pharmacologic properties of crystalloid, colloidal, and oxygen-carrying resuscitation fluids are described, and the findings of clinical trials of these solutions are discussed. Fluid administration is a fundamental part of resuscitation therapy. Crystalloid solutions supply water and sodium to maintain the osmotic gradient between the extravascular and intravascular compartments. Examples are lactated Ringer's injection and 0.9% sodium chloride injection. Colloidal solutions, such as those containing albumin, dextrans, or starches, increase the plasma oncotic pressure and effectively move fluid from the interstitial compartment to the plasma compartment. Oxygen-carrying resuscitation fluids, such as whole blood and artificial hemoglobin solutions, not only increase plasma volume but improve tissue oxygenation. Clinically, colloidal solutions are generally superior to crystalloids in their ability to expand plasma volume. However, colloids may impair coagulation, interfere with organ function, and cause anaphylactoid reactions. Crystalloid solutions represent the least expensive option and are less likely to promote bleeding, but they are more likely to cause edema because larger volumes are needed. Favorable experience with inexpensive hypertonic crystalloids with improved plasma volume expansion properties may favor a return to resuscitation with crystalloid solutions. Oxygen-carrying resuscitation fluids are indicated when the patient has lost more than 25% of the total blood volume. Tailoring therapy to the individual patient and close monitoring are essential to safe and effective fluid resuscitation.