In this chapter, we present the security architecture of the Java ME CLDC platform. The security architecture will be presented by making a distinction between the security of the Connected Limited Device Configuration (CLDC) and the security of the Mobile Information Device Profile (MIDP). The reason behind this distinction is that, while both CLDC and MIDP implement security mechanisms, each component is responsible for fulfilling specific security requirements. We start this chapter by recalling the basis of Java security.