DescriptionSmart phones are increasingly being equipped with operating systems that compare in complexity with those on desktop computers. This trend makes smart phone operating systems vulnerable to many of the same threats as desktop operating systems. In this dissertation, we focus on the threat posed by smart phone rootkits. Rootkits are malware that stealthily modify operating system code and data to achieve malicious goals, and have long been a problem for desktops. We use four example rootkits to show that smart phones are just as vulnerable to rootkits as desktop operating systems. However, the ubiquity of smart phones and the unique interfaces that they expose, such as voice, GPS and battery, make the social consequences of rootkits particularly devastating. The rapid growth of mobile malware and the four rootkit attacks developed, necessitates the presence of robust malware detectors on mobile devices. However, running malware detectors on mobile devices may drain their battery, causing users to disable these protection mechanisms to save power. This dissertation studies the security versus energy tradeoffs for a particularly challenging class of malware detectors, namely rootkit detectors. We investigate the security versus energy tradeoffs along two axes: attack surface and malware scanning frequency, for both code and data based rootkit detectors. Our findings, based on a real implementation on a mobile handheld device, reveal that protecting against code-driven attacks is relatively cheap, while protecting against all data-driven attacks is prohibitively expensive. Based on our findings, we determine a sweet spot in the security versus energy tradeoff, called the balanced profile, which protects a mobile device against a vast majority of known attacks, while consuming a limited amount of extra battery power.