Theranostics, or the combination of therapy and diagnostic imaging in a single platform, are a booming field of research in the 21st century. Theranostics have been widely investigated across biomedical imaging modalities as they show great promise for image-guided drug delivery, non-invasive treatment monitoring and personalized medicine. Examples of magnetic resonance imaging (MRI) theranostics are extensive in the literature, owing to the technique’s tunable soft-tissue contrast, limitless depth penetration, high resolution, and lack of ionizing radiation. This chapter will provide an overview of gadolinium(III)-based MRI theranostics, and evaluate both small molecule and nanomaterial strategies. A discussion of nanomaterial platforms will be divided into subcategories: metal-based nanomaterials encompassing metal oxides, metal sulfides, gold nanostructures, and lanthanide upconversion nanoparticles; carbon nanomaterials; mesoporous silica nanoparticles; dendrimers and polymers; and biomolecular nanomaterials.While therapeutic and targeting strategies will predominantly focus on cancer, examples featuring other disease states will be highlighted. An overview of magnetic particle imaging (MPI) theranostics will also be included, as this emerging area holds significant biomedical promise.