Vascular access is an important component of therapy of the critical patient, in particular for fluid resuscitation in cases of circulatory collapse, or during cardiopulmonary resuscitation. Intravenous access is well described in companion animal medicine, but becomes increasingly more difficult with decreasing patient size. Intraosseous catheterization provides vascular access in these patients for which intravenous catheterization is difficult or impractical. Intraosseous catheterization can be accomplished in avian, exotic mammal, and some reptile patients. Reports of serious complications are rare. Copyright 2008 Elsevier Inc. All rights reserved.