Osteopontin (OPN) is a soluble secreted phosphoprotein that binds with high affinity to several different integrins. While numerous activities have been ascribed to OPN in vitro, and several in vivo functions have been suggested for the molecule, including much attention focused recently on OPN in different pathologies, the actual role that the protein plays in mammalian physiology remains conjectural. Analysis of recently developed strains of mice lacking OPN because of a targeted disruption of its gene promises to provide important information on this issue. Here, we review the data implicating OPN as functioning in a variety of pathologic conditions as well as the initial results generated with the OPN knockout mice, with particular focus on the question of whether OPN has a protective or a damaging effect in different pathologies. These data will be important to the definition of how OPN contributes to mammalian physiology and pathophysiology.