This paper refers to the new planned Western Extension of the Athens Metro to Piraeus city. The railway consists of a 9.45m diameter and 8.2 km length tunnel and 7 stations. Tunnelling works are expected to be undertaken within a variety of lithological formations ranging from very strong alpine limestones to recent soft littoral deposits. The tunnel alignment was divided in thirteen areas, with respect to the geological and geotechnical conditions that may be encountered during construction. In many of these zones, the geotechnical conditions in conjunction with the presence of sensitive surface and/or subsurface structures led to the selection of a closed-face TBM (EPBM or Slurry TBM) as the appropriate tunnelling technique. The Piraeus extension is a project where both types of commonly used closed-face tunnelling machines will encounter ‘text book’ application areas yet not always. The applicability of each type of TBM is discussed using the available data obtained from an extensive site investigation campaign.