The performance analysis of double shield TBMs in difficult ground conditions in the Gerede tunnel is presented in this study. The strength of the encountered formations along the tunnel route varied from medium strength (sandstone, limestone) to high strength (basalt). The total length of tunnels is 31.6 km, which was excavated by three double shield TBMs having diameter of 5.57 m. Literature studies are first carried out in order to review the difficult ground conditions and their impacts on mechanized tunnelling. Later, the project, geology, and the characteristics of the TBMs are given in detail. Then the factors affecting the performance of the TBMs, machine utilization, and operational parameters (torque, thrust) are discussed in detail. In the light of these facts, the main objective of this study is to describe the possible improvement methods to reduce the effect of these difficulties on TBM performance.