With its exquisite anatomical resolution and wide-ranging functional imaging capabilities, magnetic resonance imaging (MRI) has found multiple applications in detection, staging, and monitoring treatment response in cancer. The metabolic information provided by magnetic resonance spectroscopy (MRS) is being actively investigated to complement MRI parameters, as well as existing biomarkers, in cancer detection and in monitoring response to treatment. Located at the interface of detection and therapy, theranostic imaging is a rapidly expanding new field that is showing significant promise for precision medicine of cancer. Innovations in the development of novel nanoparticles decorated with imaging reporters that can be used to deliver therapeutic cargo to specific cells and environments have provided new roles for MRI and MRS in theranostic imaging.