Electrical Connector System

High-speed backplane connectors systems for mounting a substrate that are capable of operating at speeds of up to at least 25 Gbps, while in some implementations also providing pin densities of at least 50 pairs of electrical connectors per inch are disclosed. Implementations of the high-speed connector systems may provide ground shields and/or other ground structures that substantially encapsulate electrical connector pairs, which may be differential electrical connector pairs, in a three-dimensional manner throughout a backplane footprint, a backplane connector, and a daughtercard footprint. These encapsulating ground shields and/or ground structures prevent undesirable propagation of non-traverse, longitudinal, and higher-order modes when the high-speed backplane connector systems operates at frequencies up to at least 30 GHz.