In this chapter we elaborate on the need for security frameworks at the link-layer and describe what services they provide to the upper layers. We review the proposed frameworks in the bibliography and discuss about their pros and cons. Then we present in more detail the design and implementation of one of them, the L³Sec framework, in order to show what issues arise in such a process and how they can be solved. Some of these features include providing acceptable resistance against node capture attacks and replay attacks, as well as the run-time composition of security services in a completely transparent way. The framework is able to satisfy its requirements based on an an ef.cient scalable post-distribution key management scheme, which we also present.