Dual-modal in vivo tumor imaging and photodynamic therapy using hexagonal NaYF4:Yb,Er/NaGdF4 core–shell upconverting nanoparticles combined with a photosensitizer, chlorin e6, is reported. Tumors can be clearly observed not only in the upconversion luminescence image but also in the magnetic resonance image. In vivo photodynamic therapy by systemic administration is demonstrated under 980 nm irradiation. Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Theranostic Probe Based on Lanthanide‐Doped Nanoparticles for Simultaneous In Vivo Dual‐Modal Imaging and Photodynamic Therapy