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Direct measurement of in-situ stress magnitudes at mid-crustal depth is one of the primary motivations of deep scientific drilling programs. In this paper we discuss the potential for making such measurements using three methods. First, the usefulness of hydraulic fracturing in deep boreholes will be discussed. We demonstrate that extremely high breakdown and fracture extension pressures will be required at depth, especially in compressional tectonic environments. Second, as wellbore breakouts are likely to occur in deep boreholes and could make hydraulic fracturing extremely difficult, the potential for estimating stress magnitudes through analysis of the breakouts will be discussed. Breakouts reported in the Kola Penninsula hole in the Soviet Union will be analysed and the magnitudes of in-situ stresses estimated to depths of 11.6 km. Finally, a new stress measurement method will be …