The number of circulating stem cells and progenitor cells can be increased by physiological stress, such as exercise, stress, and infections. The process of shifting the stem cells from the bone marrow into the peripheral blood is referred to as "mobilization" or "egress." Cytokine-mobilized hematopoietic progenitor cells (HPCs) are currently used for autologous or allogeneic stem cell transplantation in a variety of malignant and nonmalignant diseases. In spite of the wide-spread use of mobilized peripheral blood stem cells for transplantation, the mechanisms underlying mobilization are still incompletely understood. Here we discuss the role of neutrophils and proteases as mediators of stem cell mobilization.