Recent technical papers have demonstrated development flexibility, and the potential for increased ultimate hydrocarbon recovery resulting from the application of extended reach and horizontal drilling techniques. Serious mechanical borehole stability problems, principally borehole collapse and tensile fracturing, can be associated with the drilling of such wells. The threshold drilling fluid pressures for the onset of each mechanism depend on the earth stresses and the mechanical properties of the formations. Hence, there is the need for careful engineering assessments and planning prior to drilling these wells. In this paper the methods of approach used in determining the magnitudes of the principal in-situ stresses by collating geological, drilling, leak-off, logging, well test, mini-frac and laboratory test data are presented. The results formed the basis for a detailed assessment of the mechanical stability of a planned horizontal well in UKCS Block 9/13. This horizontal well was later successfully drilled from a semi-submersible drilling rig, utilising carefully tailored drilling fluid densities based on borehole stability analysis.