The introduction of hybrid positron emission/magnetic resonance tomography (PET/MR) in diagnostic clinical imaging was a major step in the evolution of ever-more sophisticated imaging systems combining two strategies formerly regarded as technically incompatible in a single device. The advent of PET/MR opened up many new avenues in clinical and research environments, mainly by providing multi-modality images obtained during a single examination. Ideally, simultaneous data acquisition with hybrid PET/MR should warrant exact image co-registration of all multi-modality image volumes provided by both systems. This assumes that there is negligible mutual electronic, technical and logistical interference on the respective simultaneous measurements. Recently, such hybrid dedicated head and whole-body systems were successfully applied in an increasing number of cases. When employed for brain imaging, PET/MR has the potential to provide high-resolution multi-modality datasets. However, it also demands careful consideration of the multitude of features offered, as well as the limitations. There are open issues that have to be considered, such as the handling of patient motion during extended periods of data acquisition, optimized sampling of derived images to ease the visual interpretation and quantitative evaluation of co-registered images. This paper will briefly summarize the current status of PET/MR within the framework of developments for image co-registration and discuss current limitations and future perspectives.