The Kowloon Southern Link (KSL) is a 3.8 km underground railway project in a busy urban area with complex geological and alignment constraints.A key challenge for the project is to construct a critical section of the tunnels above the existing MTR tunnels and below a heavily trafficked road junction, which is termed as the MTR Crossing. Cut and cover method was originally envisaged to construct the MTR Crossing. In view of the complex ground conditions and other physical site constraints, the contractor proposed to extend the bored tunnels using a tunnel boring machine (TBM) across the existing MTR tunnels as an alternative. Adopting the TBM method for the MTR crossing introduced specific challenges which called for comprehensive engineering studies, extensive ground treatment, tailored protective measure for the MTR tunnels, as well as sophisticated instrumentation in order to ensure that the railway operations and the public safety are not compromised. This paper is to discuss the design development of the scheme and the methodology of the MTR Crossing.