In this paper, the authors present various distributed Intrusion Detection System (IDS), based on mobile agents, that detects intrusion from outside the network segment as well as from inside. Mobile agents are intelligent agents that can migrate among hosts. They can execute tasks autonomously in dynamic environments. Besides general definitions of these IDS system architectures, it includes an overview of several network and agent based intrusion detection systems. The system shows a superior performance compared to central sniffing IDS techniques, and saves network resources compared to other distributed IDSs.