The design and construction of the Medway immersed tube tunnel and approach structures required a thorough investigation of the site and a detailed understanding of the geology and geotechnical behaviour of the soils. The paper describes the ground investigation and discusses how the design accommodated the ground conditions to allow the tunnel and approach structures to be safely and efficiently constructed. Two main geotechnical challenges are described; the approach embankments and the casting basin in which the immersed tube tunnel sections were constructed. Part of the approach embankment was constructed over very soft soils. The construction made use of vertical drains, stage construction and preloading to ensure that the embankment was constructed economically within the available programme. The immersed tube tunnel sections were constructed in a casting basin next to the river. The design of the casting basin required careful consideration of the stability of the temporary slopes and the dewatering system had to deal with large quantities of water without causing significant settlement of some sensitive nearby structures. These two areas are discussed and details of the construction and performance are provided.