Although most arguments supporting preferential tax treatment of electronic commerce are suspect, the existence of network externalities provides one potentially defensible rationale. This paper considers (1) direct network externalities, which arise in communications networks like the Internet, (2) indirect network externalities, which arise in computer hardware/software systems in imperfectly competitive markets, and (3) learning network externalities, which arise when existing network participants assist new users. It concludes the case for preferential treatment is weak, and that proposed blanket sales tax exemptions of remote (or all) e-commerce are especially inappropriate. Finally, the paper comments briefly on US sales tax reform efforts.