Stereotactic radiosurgery (SRS) implies the delivery of a single, high dose of radiation using stereotactic techniques. Classically, for cranial SRS, a rigid neurosurgical frame is attached to the patient’s skull, and a stereotactic localizing device is attached to the frame to allow for improved targeting accuracy. Commercial SRS systems are available in which immobilization is achieved without a rigid neurosurgical frame. While there are innumerable commercial SRS and stereotactic radiation therapy systems, all of these systems utilize the same underlying principles: (1) patient immobilization, (2) targeting accuracy, (3) delivery of high doses of radiation, and (4) heterogeneous dose distribution with a steep dose gradient. Commercial SRS systems include Gamma Knife, which utilizes 201 radioactive cobalt sources positioned in a semispherical array, and Cyberknife, which utilizes robotics technology allowing the linear accelerator (LINAC) to track the patient position in real time during treatment. A standard LINAC can be equipped with cones attached to its head, allowing for circular collimation of the beam. The dose distribution from Gamma Knife, Cyberknife, or a standard LINAC equipped with cones is spherical, and thus treating nonspherically shaped targets requires the superposition of multiple spherical dose distributions. A LINAC equipped with multi-leaf collimators can also be used for SRS, which allows more conformal beam shaping. Also called radiation surgery, radiosurgery, and stereotaxic radiosurgery.