With the advent of wireless and mobile networks, the Internet is rapidly evolving from a set of connected stationary machines to include mobile handheld devices. This creates new opportunities for customers to conduct business from any location at any time. However, the electronic commerce technologies currently used cannot be applied directly since most were developed based on fixed, wired networks. As a result, a new research area, mobile commerce, is now being developed to supplement existing electronic commerce capabilities. This chapter discusses the security issues related to this new field, along with possible countermeasures, and introduces a mobile agent based solution for mobile commerce.