Intrusion Detection Systems (IDSs) play an important role in the defense strategy of site security officers. An IDS can act as a second line of defense to provide security analysts with the necessary insights into the nature of hostile activities. Therefore, a good understanding of IDSs helps administrators make informed decisions when it comes to choosing the right product for their systems. System programmers will appreciate a classification of the different IDS design and implementation approaches that highlight their practical use. One can never assess an intrusion detection system without knowing the performance measurements involved and the evaluations used to gauge these metrics. This chapter covers to a reasonable extent the above mentioned issues and draws some conclusions.