Rapid developments in imaging and radiation-delivery technology have fueled the application of small photon beams in stereotactic radiation therapy (SRT). Historically, stereotaxy referred to the use of a three-dimensional coordinate system to localize intracranial targets and has been more recently extensively developed in extracranial clinical situations. SRT involves stereotactic localization techniques combined with the delivery of multiple small photon fields in a few high-dose fractions. In SRT, the therapeutic ratio is optimized through delivery of highly conformal absorbed dose distributions with steep dose fall-off ensuring optimal absorbed dose in the target volume combined with minimal normal-tissue irradiation. Consistent with previous ICRU Reports 50 (ICRU, 1993), 62, (ICRU, 1999), and 83, (ICRU, 2010), this Report recommends a strict definition of target volumes (GTV, CTV) by reviewing imaging modalities used in clinical practice. This Report covers fundamentals of small-field dosimetry, treatment-planning algorithms, commissioning, and quality assurance for the existing delivery systems, as well as the role of image guidance during delivery. Finally, it recommends a framework for prescribing, recording, and reporting stereotactic radiotherapy, and covers most of the pathologies eligible for stereotactic delivery (malignant and non-malignant).