Introduction: Virtual surgical planning is often used to prepare mandibular segment resection with subsequent reconstruction. Patient-specific cutting guides translate the planned resection and reconstruction osteotomies to the surgery. The accuracy of computer-assisted surgery is currently evaluated by heterogeneous methodologies for post-operative imaging, segmentation, registration, and accuracy measurements. Objective: This thesis aims to develop an objective, reproducible and insightful evaluation methodology. The evaluation methodology will compare planned osteotomies with actual osteotomies in segmental mandibular resection and reconstruction. The designed methodology will be applied in a retrospective study. Method: Actual osteotomies were defined by a plane fitted through manually defined points on the post-operative imaging. The actual osteotomies were aligned to the pre-operative mandible or fibula model. Distance and angular deviation were measured between the planned and actual osteotomies. Resection osteotomy distance deviation was defined as the distance between the centre of mass of the actual and planned intersection of the pre-operative mandible model and the osteotomies. The maximum distance between the intersections was also measured. Reconstruction osteotomy distance deviation was defined as the length difference between the planned and actual fibula segments. Angular deviation of the resection and reconstruction osteotomies were defined by two angular differences based on the saw slot, i.e. the angle across the saw slot (x-axis) and the angle through the saw slot (y-axis). Results: A semi-automatic novel methodology was developed. The intra-observer variation of the osteotomy localisation was ± 0.4 mm for distance deviation and ± 2.1° for the angular deviation. The inter-observer variation was ± 0.8 mm for the distance deviation and ± 2.4° for the angular deviation. Sixteen patients were included in the retrospective study. For the resection osteotomies, the absolute average distance deviation was 2.1 ± 1.9 mm for the centre of mass and 3.1 ± 2.3 mm for the maximum distance. The fibular segments differed by 2.4 ± 2.5 mm in length. Angular deviations around the x-axis were 3.7 ± 3.4° for resection and 6.9 ± 7.1° for reconstruction osteotomies, and deviations around the y-axis were 5.7 ± 5.8°and 9.1 ± 11.4°, respectively. Conclusion: The evaluation methodology provides guidelines for post-operative imaging, segmentation, osteotomy localisation, registration, and osteotomy comparison. The difference in distance deviated was within an absolute average of 3 mm. The angular deviation was significantly larger for the reconstruction osteotomies than for resection osteotomies, requiring further research.