Well-designed security mechanisms intended to protect privileged data or resources typically rely on existence of some sort of secret that is known only to the intended users of that mechanism. In general, the secret is called the key, even though it can be as simple as a memorized password. Many times, the strength of a security mechanism greatly depends on how the key/s, used by that mechanism, are generated and handled. The fact that, the US National Institute of Standards and Technology (NIST) has held several key management workshops, emphasizes the importance of key management mechanisms. As a result of these workshops, NIST has produced a set of guidelines provided in documentations such as [KMGNIST1]. The NIST documentations are very important, especially since compliance to government standards is a necessary requirement for many security products being deployed for government and public safety agencies. Since this book is not a text on cryptography (and we do not claim that we can write one!), we suffice devoting this chapter to main key management concepts and mechanisms without going into the details of the mathematics involved. We will provide more detail on protocols deploying these concepts in Chapter 4 when describing security mechanisms for the Internet and in Chapter 9 when describing infrastructure for managing public key certificates.