The cytokine and cell attachment protein osteopontin (OPN) is not necessary for the development and survival of mice in a clean animal facility. The primary role of OPN appears to be that of facilitating recovery of the organism after injury or infection, which generally causes an increase in its expression. It also is essential for some forms of bone remodeling. OPN stimulates cellular signaling pathways via various receptors found on most cell types and can encourage cell migration. OPN modulates immune and inflammatory responses and possibly negatively regulates Ras signaling pathways. Its apparent ability to enhance cell survival by inhibiting apoptosis may explain why the metastatic proficiency of tumor cells increases with increased OPN expression. J. Cell. Biochem. Suppls. 30/31:92–102, 1998. © 1998 Wiley‐Liss, Inc.