PET and MRI are powerful imaging techniques that have been used extensively to study diseases of the brain, head, and neck. In many diseases, including dementia, epilepsy and head and neck cancers, PET and MRI play important and complementary roles in standard diagnostic evaluation. Recent advances in PET detector technology have allowed combination of PET and MRI into a single machine capable of simultaneous imaging which may offer several, distinct advantages over serial imaging and post-acquisition fusion such as decreased patient burden and improved PET imaging quantification. In addition, a PET/MR instrument potentially has the unique ability to combine time-dependent physiological and functional information from MRI with the specific metabolic and receptor specific information from PET imaging. Over the past several years, numerous anecdotal reports and several larger studies showed the feasibility of PET/MR in various brain and head and neck applications including, for example, concomitant fMRI and PET neuroreceptor imaging. Future well-designed studies with a focus on evaluating and optimizing the synergistic qualities of these combined technologies may fulfill the promise of PET-MRI to provide unparalleled opportunities to understand complex brain function and pathology.