The family of related cell surface adhesion molecules designated CD44 are multifunctional proteins displayed by a wide variety of normal and malignant tissues. CD44 properties in vitro include hyaluronate-mediated adhesion, motility, hyaluronate degradation, self-aggregation, and adhesion to lymphoid tissue. These properties are among several that are required by invasive and metastatic tumor cells. Several in vivo experiments indicate that tumor cells transfected to overexpress specific CD44 isoforms display an enhancement in metastatic potential. Numerous CD44 isoforms exist as a result of alternative splicing, and specific isoforms enhance tumor cell metastasis more efficiently than others. This suggests that regulation of CD44 alternative splicing is an important determinant of metastatic potential. Several human tumors display specific alterations in CD44 isoform expression, and the extent of clinical disease in specific tumors correlates with the CD44 isoform expression pattern. In this review we analyze these data that suggest that CD44 plays an important role in tumor metastasis.