Work on the first phase of the Hong Kong Mass Transit Railway (known as the Modified Initial System) started in November 1975 and the line was opened to the public in February 1980. It is 15.6 km in length and runs from the Central District of Hong Kong Island to Kwun Tong in mainland Kowloon. The system includes 12 underground stations, three overhead stations and a maintenance and repair depot covering an area of 16.5 hectares. At the depot site a medium density high rise residential development is being constructed above podium level, providing housing and facilities for 25,000 people. The railway runs predominantly in bored tunnel, but includes short cut and cover and overhead sections, whilst the 1.4 km long harbour crossing is an immersed tube. The underground stations are box structures constructed by cut and cover methods. This paper describes the geotechnical aspects of the project, including the geology of the route, preliminary investigations, construction methods (some of which were novel to this part of the Far East), geotechnical instrumentation and the effects of the work on the surrounding high density urban environment.