The Medway Viaduct carries the new high speed Channel Tunnel Rail Link over the Medway River upstream of Rochester and is part of the new high-speed railway line between London and the English Channel, Section 1 of which is due to open this year. The viaduct is a multi-span structure, 1.2km long, with typical approach spans of 40.5m, spanning the Medway River with a central navigation span of 152m. The viaduct substructure is reinforced concrete pier columns supported on bored piles. This paper describes the design development, including the substructure and superstructure design, and includes some particular design features e.g. the prestressing and articulation systems, movement joints and trackwork furniture.