Break-in and break-out operations represent critical construction phases in shield tunnelling, particularly in water-bearing and heterogeneous ground conditions. During these transitions, temporary modifications in confinement and hydraulic boundary conditions disturb the equilibrium between face support pressure and in-situ stresses, increasing the risk of ground instability and groundwater inflow. The paper discusses the mechanical behavior of the ground during TBM breakthrough through diaphragm walls and station boxes, with emphasis on operational pressure control, annular gap sealing and monitoring requirements. Selected engineering solutions applied to mitigate breakthrough risks are presented, including sealing rings, cement concrete blocks and ground improvement techniques. Their structural configuration, operational role and limitations are analyzed in relation to groundwater pressure and soil permeability. Practical implementation is illustrated through case studies from metro projects, including the FGC Terrassa extension and Sofia Metro Line 3, where combined mechanical and geotechnical measures were adopted to ensure safe TBM transition under low cover and high groundwater conditions. The presented examples confirm that successful breakthrough operations require integrated design combining structural solutions, ground treatment and strict operational control. Break-in and break-out should therefore be treated as governing design stages in tunnel portal engineering rather than routine construction steps.