This chapter examines the scope of malicious software (malware) threats to mobile devices. The stakes for the wireless industry are high. While malware is rampant among 1 billion PCs; approximately twice as many mobile users currently enjoy a malware-free experience. However; since the appearance of the Cabir worm in 2004; malware for mobile devices has evolved relatively quickly; targeted mostly at the popular Symbian smartphone platform. Significant highlights in malware evolution are pointed out that suggest that mobile devices are attracting more sophisticated malware attacks. Fortunately; a range of host-based and network-based defenses have been developed from decades of experience with PC malware. Activities are underway to improve protection of mobile devices before the malware problem becomes catastrophic; but developers are limited by the capabilities of handheld devices.