Osteopontin (OPN) is a calcium-binding phosphoprotein which is believed to play a role in several different and apparently distinct cellular processes. Recently, expression of OPN has been linked to tumorigenesis and metastasis in several experimental animal models and human patient studies. Precisely what role OPN plays in these processes is far from clear. OPN is known to importantly contribute to cell adhesion interactions, possibly mediated by the highly conserved GRGDS amino acid sequence, a motif found on a number of proteins which play a role in cell adhesion. In addition, OPN has binding affinity for several different cellular receptors, potentially allowing it to stimulate various signaling pathways and influence distinct cellular events which may ultimately favor tumorigenesis or metastasis.