This chapter describes the key elements in detail and highlights the investigation necessary for tunnel construction. Tunnel construction is governed by the ground and hence a thorough site investigation is vital to obtain ground characteristics and geotechnical parameters. A typical site investigation comprises four key elements: the desk study, site reconnaissance, ground investigation and the production of the site investigation report(s). The desk study is a very important stage of any site investigation, which, if done well, can save considerable time, and hence money, later on in the investigation process. Site reconnaissance is the first site-specific work. With tunnelling projects, it is rarely possible to walk along the entire length of the tunnel alignment, but this should be attempted as it can provide excellent detailed site knowledge for future planning. The ground investigation element of the site investigation should be planned based on the findings of the desk study and the site reconnaissance.