The introduction of FDG whole body imaging with PET/CT by Townsend, Beyer & Nutt around the year 2000 has resulted in a transformational change in cancer management. Initial staging, characterisation of primary lesions, monitoring of response to therapy and re-staging are all now regular, accepted uses of FDG imaging in a number of malignancies. The key features that drive the use of PET/CT are the high sensitivity for detection of nodal and malignant spread of cancer, the assessment of the whole body rather than just a regional study of where cancer might be suspected, the ability to detect early changes after treatment (either disease regression or progression) and the rapid patient throughput achievable. In short, rapid FDG whole body PET/CT is a technology which is hard to surpass in many instances. The combination of PET with MRI, introduced around the year 2010, therefore needs to provide something different to that available from PET/CT for most patients investigations, except perhaps for paediatric patients where the radiation dose from many PET/CT examinations may be undesirable. Therefore, PET/MRI should be explored in areas that conventional FDG PET/CT may not provide definitive information. This will see PET/MRI using new tracers (labelled with F-18, Ga-68, Cu-64, Zr-89 and I-124) and in diseases that traditional PET/CT has not had a major impact, include cancers of some organs (pancreatic, liver, brain) and other disorders such as those of metabolism, neurodegeneration, cardiovascular and inflammatory conditions. Set in this context, PET/MRI has the potential to offer not just structure/function characterisation, but structure/function/tissue characterisation in a way that conventional CT combined with PET cannot provide.