This paper overviews Yttrium-90 (90Y) as a theranostic and nuclear medicine imaging of 90Y radioactivity with bremsstrahlung imaging and positron emission tomography. In addition, detection and optical imaging of 90Y radioactivity using Cerenkov luminescence will also be reviewed. Methods and approaches for qualitative and quantitative 90Y imaging will be briefly discussed. Although challenges remain for 90Y imaging, continued clinical demand for predictive imaging response assessment and target/nontarget dosimetry will drive research and technical innovation to provide greater clinical utility of 90Y as a theranostic agent.