This paper presents the intrusion detection toolkit (IDtk), an information visualization tool for intrusion detection (ID). IDtk was developed through a user-centered design process, in which we identified design guidelines to support ID users. ID analysts protect their networks by searching for evidence of attacks in ID system output, firewall and system logs, and other complex, textual data sources. Monitoring and analyzing these sources incurs a heavy cognitive load for analysts. The use of information visualization techniques offers a valuable addition to the toolkit of the ID analyst. Several visualization techniques for ID have been developed, but few usability or field studies have been completed to assess the needs of ID analysts and the usability and usefulness of these tools. We intended to fill this gap by applying a user-centered design process in the development and evaluation of IDtk, a 3D, glyph-based visualization tool that gives the user maximum flexibility in setting up how the visualization display represents ID data. The user can also customize whether the display is a simple, high-level overview to support monitoring, or a more complex 3D view allowing for viewing the data from multiple angles and thus supporting analysis and diagnosis. This flexibility was found crucial in our usability evaluation. In addition to describing the tool, we report the findings of our user evaluation and propose new guidelines for the design of information visualization tools for ID.