Stereotactic radiosurgery treatment principles and irradiation techniques have shown little evolution since its introduction in 1968. Conversely, technology progress linked to computers has produced a major impact on the methods used for treatment planning and dose delivery. In order to fully comprehend modern radiosurgery approaches, one has to acquire good insight of the underlying technology, specifically computer software. In this chapter, we describe the evolution from X-ray films to high-resolution digital imaging, the shift from simple trigonometric calculation to highly complex algorithms and new perspectives in patient follow-up. If these changes open new prospects, they also add complexity, which leads to new pitfalls and limits of the stereotactic radiosurgery method.