Intel^® Virtualization Technology^Δ for Directed I/O (VT-d) is the next important step toward comprehensive hardware support for the virtualization of Intel^® platforms. VT-d extends Intel's Virtualization Technology (VT) roadmap from existing support for IA-32 (VT-x) [1] and Itanium^® processor (VT-i) [2] virtualization to include new support for I/O- device virtualization. This paper surveys a variety of established and emerging techniques for I/O virtualization and outlines their associated problems and challenges. We then detail the architecture of VT-d and describe how it enables the industry to meet the future challenges of I/O virtualization.