Novel physiologic MRI and metabolic PET imaging provides the ability to analyze tumor tissue properties including tumor vasculature, vascular permeability, tumor cellularity, hypoxia, and tumor proliferation. Stereotactic radiosurgery (SRS) involves the delivery of a very precise, focal dose of radiation to a target. It is important to understand the radiological principles behind the various imaging modalities and their application in SRS. Recent advances in MR imaging have the potential to improve accuracy for target volume delineation and therefore outcome for intracranial tumors. Novel imaging techniques may also be used in subsequent posttreatment follow-up to distinguish between tumor recurrences versus nonneoplastic treatment-related changes. Imaging modalities discussed in this chapter include conventional MRI, MR diffusion-weighted imaging, MR perfusion imaging, dynamic contrast-enhanced MRI, MR spectroscopy, metabolic PET, and cerebral angiography. This chapter illustrates the utility of these imaging modalities in the SRS treatment and posttreatment follow-up of both malignant and benign intracranial tumors including brain metastases, recurrent glioma, arteriovenous malformations, meningiomas, and vestibular schwannomas.