HyperTransport 3.10 is the best open standard communication technology for chip-to-chip interconnects. However, its extraordinary features are of little help when building midand large-scale systems because it is unable to natively scale beyond 8 computing nodes. Therefore, it must be complemented by other interconnect technologies. The HyperTransport Consortium has intensively stimulated discussions among its high-level members in order to overcome those shortcomings. The result is the High Node Count HyperTransport Specification, which defines protocol extensions to the HyperTransport I/O Link Specification Rev. 3.10 that enable HyperTransport to natively support high numbers of computing nodes, typical of large scale server clustering and High Performance Computing (HPC) applications. This extension has been carefully designed in such a way that it extends the maximum number of connected devices to a number large enough to support current and future scalability requirements, while preserving the excellent features that made HyperTransport successful and keeping full backward compatibility with it.