Stereotactic neurosurgical procedures pertain to using a three-dimensional imaging diagnostic tool to accurately define a three-dimensional Cartesian coordinate system of the patient's head. By these methods, one can precisely determine the location of the brain region of interest and plan the optimal surgical instrument pathway or trajectory to it. These kind of procedures date back to the beginning of the twentieth century when two scientists, Horseley and Clarke, first used a stereotactic device on monkeys. There are various kinds of stereotactic devices. Conventional stereotaxy pertains to fixating a stereotactic frame to the patient's head. Newer technologies have made it possible to perform a stereotactic procedure without a frame, so-called frameless stereotaxy. The frame is fixated to the patient's skull in such a way so that the relation of the frame to the head is constant. By doing an imaging scan one can precisely determine the coordinates of every intracranial point with reference to the frame. During the surgical procedure the instrument holder is attached to the frame according to the coordinates of the target brain region of interest and trajectory. Such an approach enables a minimally invasive surgical procedure that had been precisely planned before it even started. The most relevant indications include stereotactic biopsy and deep brain stimulation. The structures of interest are very tiny. Furthermore, during the procedure the surgeon does not directly visualize the location of the instrument, they aim for the set coordinates. Hence, it is very important for a neurosurgeon, when using a stereotactic device for a certain procedure, to be fully aware of its precision and reliability which means to understand to what extent do the target coordinates and the planned trajectory correspond to the actual target location and instrument pathway during the procedure. The total clinically relevant error comprises errors associated with each procedural step, including imaging, target selection, trajectory calculations, and the mechanical errors of stereotactic frames.