CD9 has been described in the literature for almost two decades yet its structure was only identified within the last few years. CD9 belongs to the tetraspanin family of proteins that is also referred to as the Transmembrane 4 (TM4) Superfamily. This chapter describes the functional characterization of CD9 in platelets as well as in other cell types. AntiCD9 mAbs activate platelets via crosslinking of CD9 and the FcγRII receptor. Evidence suggests that CD9 may be in complex with the adhesive protein receptors, the integrins, and function in mechanisms associated with cell spreading and movement.