The recruitment of cells to sites of inflammation is an essential component of the host inflammatory response. Cell recruitment relies on the coordinated action of cell activation, cell adhesion, chemoattraction, and transmigration across the endothelial barrier. The chemokines (chemotactic cytokines) are a newly described superfamily of secreted proteins that play an important role in cellular chemotaxis. In this chapter we will review chemokine structure, emphasizing how certain structural elements confer differential functional activity. We will also discuss chemokine receptors, their cellular expression, and their intracellular signaling mechanisms. The role of chemokines in cell trafficking will be reviewed in terms of their role in homeostasis and development and their participation in cell recruitment to sites of inflammation. Illustrative examples of chemokine action in vivo will be drawn from specific human diseases and animals models.