Mobile and handheld devices are becoming an integral part of people’s work, life and entertainment. These lightweight pocket-sized devices offer great mobility, acceptable computation power and friendly user interfaces. As people are making business transactions and managing their online bank accounts via handheld devices, they are concerned with the security level that mobile devices and systems provide. In this chapter we will discuss whether these devices, equipped with very limited computation power compared to full-sized computers, can make equivalent security services available to users. We focus on the security designs and technologies of hardware, operating systems and applications for mobile and handheld devices.