The intracellular segments of classic adhesion molecules such as N-CAM do not show structural similarity to any known signaling molecules. This suggests that their effects on signaling responses must be exerted indirectly through associated proteins. In contrast, many receptor protein tyrosine phosphatases (RPTPs) possess extracellular segments with homology to cell adhesion molecules linked directly to intracellular segments comprising one or two protein tyrosine phosphatase catalytic domains. Therefore, the RPTPs have the potential for direct modulation of catalytic function through engagement of the extracellular segment, suggesting they could be direct signal transducers of cell contact phenomena. In the past few years, some RPTPs have been shown to effect cell-cell adhesion directly via homophilic binding or indirectly by association with known cell adhesion molecules. In addition, RPTPs have been localized to points of cell-cell or cell-matrix contact, indicating their potential to regulate these structures.