The introduction of Magnetic Resonance (MR) Imaging in treatment planning for radiotherapy of prostate cancer give rise to new challenges in defining the treatment volume, or the target. Imaging with MR have several advantages, especially better soft-tissue contrast, compared with the standard image modality, Computed Tomography (CT). The purpose of this project was to determine how the target definition varies with the choice of image modality and the systematic differences between them. The purpose was also to determine how the inter- and intra physician variability influences the delineation, depending on which image modality that is used. In the project, five physicians delineated the prostate gland on CT and MR images for nine patients. The physicians had no information of which image series that was connected, and were thus delineating independent. After the delineation, the CT and MR image series was set in the same geometrical coordinate system in the treatment planning system Oncentra. The target delineations were analysed by comparing the radial distances from the centre of mass in different directions, such as anterior, posterior, etc. The radial distances were later used to evaluate the variability of the delineations and to determine the inter and intra physician variability in different directions of the targets. ANOVA was also preformed to determine if there is a significant difference between the parameters, as the image modality and the image modalities influence on the physicians delineations. A model was made to investigate how the MR delineations differ from a CT defined volume that 95% of the delineations cover, called the ideal CT delineation. From this, the median deviation in different directions  was analysed and it was found that the median value of the MR delineations in different directions was between -0.10-2.27 mm larger than the ideal CT delineation. The fluctuations between the delineations was, however, large. The target volume was larger for CT defined volumes in 87% of the cases, compared with MR targets. The inter physician variability was found to be between 0.54-2.17 mm for the CT based delineations and 0.68-2.08 mm for the MR based target delineations. The intra physician variability was larger than the inter physician variability, between 0.74-2.51 mm for CT based delineations and 0.85-1.45 mm for MR based delineations. The median variability of the delineations were not uniform around the target volume but were larger for example in the superior and inferior directions and had its minimum in the posterior direction. The ANOVA tests showed a significant difference between MR and CT based target delineations, it also shows a relation between the delineating physician and the image modality, meaning that the physicians are delineating different on CT and MR images. Target volumes defined on MR images are in general smaller than the CT defined targets. The soft tissue contrast in MR images makes the delineating process easier, however, the analysis of the variability in this project indicates that the variations of MR based target delineations are larger than the CT based. The large variation of the delineations implies that clinical tests should be made to ensure a proper dose coverage before MR could be used as the only image modality in radiotherapy treatment planning of prostate cancer.