This chapter considers mobile e-commerce on mobile phones and in particular GSM mobile phones. It is aiming at clarifying the differences between e-commerce and mobile e-commerce. The powerful aspects of this new e-commerce form born with mobility are explained carefully. The chapter starts with a presentation of the generic business-to-consumer e-commerce followed by two typical examples of e-commerce on the World Wide Web. The mobile e-commerce is then described thoroughly. The fundamental differences with e-commerce are explained, and an ideal mobile commerce system is presented. The limitations in current wireless communication systems are clarified. The Wireless Application Protocol together with its impacts and limitations for mobile e-commerce are also considered. Different types of mobile phones will then be studied, and their limitations regarding security are shown. A solution providing adequate authentication and micro-payment, called Mobile ePay, is then presented with examples of operation. A mobile e-commerce receipt system enabling instantaneous delivery is described thoroughly. The chapter concludes with suggestions for future works.