Avastin is thought to normalize tumor vasculature and restore the blood brain barrier, resulting in less enhancement and peritumoral edema. Conventional measurements of tumor response such as the Macdonald criteria are based on the dimensions of enhancing tumor. Converting enhancing to non-enhancing tumor could therefore be considered treatment response, even in the absence of change in tumor size. But it is unknown if reduction in enhancement, by itself, correlates with patient outcome measures such as progression free and overall survival. In general change in tumor size is thought to be a more accurate biomarker of response, underscoring the importance of assessing faintly-enhancing or non-enhancing tumor burden in patients following Avastin therapy. Unfortunately non-enhancing tumor can be difficult to distinguish from other causes of increased T2 signal intensity, including peritumoral edema and radiation-induced gliosis, which are common among glioblastoma patients. This difficulty has led to efforts to find alternative ways to measure non-enhancing tumor, as well as to characterize additional biomarkers for tumor response that correlate with outcome measures and add value to standard MRI. One area of active research is the use of physiologic imaging, which has the potential to detect drug effects before change in tumor size is evident.