Borehole instability has been a central problem for drilling operations and borehole measurements within the German Continental Deep Drilling Project (KTB) main borehole, which has been drilled entirely through crystalline rocks, mostly paragneisses and metabasites. The types of borehole instabilities observed were drilling‐induced tensile fractures, breakouts, and convergence zones. These were strongly dominated by foliate, graphitic gneisses with sometimes very low and anisotropic rock strength, unfavorably high and nonuniform horizontal stresses, and the effects of hydraulic interactions of the drill mud and formation fluids. The instability problems encountered in the KTB main borehole have clearly demonstrated that undergauge sections, which first appeared at depths below 7500 m and probably resulted from time‐dependent convergence of wet crystalline rocks, had a much greater impact on the drilling operations than all the breakouts or drilling‐induced tensile fractures.