Within the past few years, it has become recognized that the immune system communicates to the brain. Substances released from activated immune cells (cytokines) stimulate peripheral nerves, thereby signaling the brain and spinal cord that infection/inflammation has occurred. Additionally, peripheral infection/inflammation leads to de novo synthesis and release of cytokines within the brain and spinal cord. Thus, cytokines effect neural activation both peripherally and centrally. Through this communication pathway, cytokines such as interleukin-1, interleukin-6 and tumor necrosis factor markedly alter brain function, physiology and behavior. One important but underrecognized aspect of this communication is the dramatic impact that immune activation has on pain modulation. The purpose of this book is to examine, for the first time, immune-to-brain communication from the viewpoint of its effect on pain processing. It is aimed both at the basic scientist and health care providers, in order to clarify the major role that substances released by immune cells play in pain modulation. This book contains chapters contributed by all of the major laboratories focused on understanding how cytokines modulate pain. These chapters provide a unique vantage point from which to examine this question, as the summarized work ranges from evolutionary approaches across diverse species, to the basics of the immune response, to the effect of cytokines on peripheral and central nervous system sites, to therapeutic potential in humans.