Chemokines comprise a superfamily of at least 46 cytokines that were initially described based on their ability to bind to 18 to 22 G protein-coupled receptors to induce the directed migration of leukocytes to sites of inflammation or injury. In addition to mediating cellular migration, chemokine/chemokine receptor pairs have been shown to affect many cellular functions, including survival, adhesion, invasion, and proliferation, and to regulate circulating chemokine levels. Most malignancies also express one or more chemokine receptors. Early studies established a role for CXCR4 and CXCR7 in mediating breast cancer metastasis, but other chemokine receptors, including CXCR3, now are implicated in several malignancies as biomarkers of tumor behavior as well as potential therapeutic targets. This review summarizes our current understanding regarding the contribution of CXCR4 and CXCR3 to tumor behavior and how receptor expression is regulated, transduces intracellular signals, and contributes at the molecular level to tumor behavior. It also describes recent therapeutic approaches that target these receptors or their ligands.