Monocyte chemoattractant proteins (MCPs) are closely related members of the CC chemokine family. All are chemoattractants for monocytes, activated T cells, and NK cells, and some MCPs also attract eosinophils. Their expression patterns suggest that these chemokines may play important roles in human diseases that are characterized by mononuclear or eosinophilic cell infiltration. In vivo analyses with neutralizing antibodies and genetically targeted mice indicate that MCP-1 in particular is a major monocyte chemoattractant, and its activity is necessary for the full pathophysiological manifestations of diseases such as atherosclerosis. Targeting the MCPs or their receptors is likely to have significant therapeutic potential.