Insulin resistance plays a major role in the pathogenesis of type 2 diabetes. Insulin regulates blood glucose levels primarily by promoting glucose uptake from the blood into multiple tissues and by suppressing glucose production from the liver. The glucose transporter, GLUT4, mediates insulin-stimulated glucose uptake in muscle and adipose tissue. Decreased GLUT4 expression in adipose tissue is a common feature of many insulin resistant states. GLUT4 expression is preserved in skeletal muscle in many insulin resistant states. However, functional defects in the intracellular trafficking and plasma membrane translocation of GLUT4 result in impaired insulin-stimulated glucose uptake in muscle. Tissue-specific genetic knockout of GLUT4 expression in adipose tissue or muscle of mice has provided new insights into the pathogenesis of insulin resistance. We recently determined that the expression of serum retinol binding protein (RBP4) is induced in adipose tissue as a consequence of decreased GLUT4 expression. We found that RBP4 is elevated in the serum of insulin resistant humans and mice. Furthermore, we found that increasing serum RBP4 levels by transgenic overexpression or by injection of purified RBP4 protein into normal mice causes insulin resistance. Therefore, RBP4 appears to play an important role in mediating adipose tissue communication with other insulin target tissues in insulin resistant states.