The objective of this Book is to develop detection techniques. This book discusses the design of intrusive rootkits. We show how these codes are innovative and malicious compared to the usual malware like viruses and Trojans, which allows us to expose a functional architecture of rootkits. We propose not only the rootkit software but also 'a communication between the attacker and his tool or interactions with the system. We find that designing rootkits are close to those of steganography. Finally, our present own Linux kernel-based rootkit, and several new techniques designed to increase secrecy. We call rootkit, a set of changes to an attacker to maintain in time a fraudulent control on an information system. The methods employed by a rootkit to fulfill its role takes different approaches using types of malicious code (logic bombs, Trojan horses, viruses, etc.).