The reason for this master thesis was a disagreement of predicted mud weight window from a geomechanical study and actual field experience. The focus was on finding a way to challenge the outcome of geomechanical studies in general and further on gaining deeper insight into the procedure of data processing and mud weight window calculation of the company which has performed the geomechanical study. The thesis describes the development of a geomechanical earth model which provides the input values for stress and mud weight window calculation. The three different stress regimes are explained as well as the development of a stress polygon. Determination and gradient computation of the three principle stresses, of pore pressure and of rock mechanical properties are discussed and supplemented with geomechanical theory. A program for stress calculation and mud weight window prediction was developed to provide the ability of verifying the results of the study. The structure and the development steps of the program are explained and the diagrams included in the program are used to continuously exemplify geomechanical theory. It was able to verify the results of the study from a calculation standpoint so further investigation is required to find the reason for the discrepancy. The company which has performed the study made the statement that underestimated rock strength could be an explanation for the inconsistency. In addition a sensitivity analysis was performed according to the input parameters which concluded with pore pressure and maximum horizontal stress to be the most important factors. Additional findings have been made throughout the thesis work which explain the reason for break outs at high mud weights in SHmax direction, the change of most preferable horizontal drilling direction in a normal faulting stress regime with change in break out width allowance and the reason for Shmin being the most preferable horizontal drilling direction in a normal faulting stress regime.