RAID 99 was the second in the series of "Research Advances in Intrusion Detection" workshops. It was held at Purdue with CERIAS as the host in 1999. The RAID International Workshop series is intended to further progress in intrusion detection by promoting the exchange of ideas in a broad range of topics among researchers, system developers, and users and by encouraging links between these groups. They bring together leading figures from academia, government, and industry to ponder the current state of intrusion detection technologies and paradigms from the research and commercial perspectives.