Internet security is getting less secure because of the existing of botnet threats. An attack plan can only be planned out to take down the botnet after the monitoring activities to understand the behaviour of a botnet. Nowadays, the architecture of the botnet is developed using Peer-to-Peer (P2P) connection causing it to be harder to be monitored and track down. This paper is mainly about existing botnet monitoring tools. The purpose of this paper is to study the ways to monitor a botnet and how monitoring mechanism works. The monitoring tools are categorized into active and passive mechanism. A crawler is an active mechanism while sensor and Honeypot are the passive mechanisms. Previous work about each mechanism is present in this paper as well.