Recently the CXCR4/CXCL12-axis has been recognized as one of the pivotal adhesion pathways by which hematopoietic stem cells are retained in the bone marrow. CXCR4 antagonists with different chemical specification are being developed. Pharmacology research guides the way to the rational development effective antagonists. One antagonist, plerixafor, is clinically approved now for stem cell mobilization of lymphoma and myeloma patients. This allows patients to receive potentially life-saving treatment which could not have been administered otherwise. Through early clinical studies it was recognized that CXCR4 antagonists also mobilize malignant hematopoetic cells, i.e. leukemia cells. In preclinical studies a sensitization of mobilized leukemic cells to standard cytotoxic chemotherapy could be shown. Clinical studies are under way. CXCR4 antagonists are an exciting new class of compounds which are also employed for the mobilization of angiogenic cells or for the treatment of solid tumors. In this book a concise review of the current status of knowledge and future developments will be presented.