At the 2024 ISMRM Annual Meeting in Singapore, a member-initiated session on MRI for biology-guided radiation therapy (RT), endorsed by the ISMRM MR in RT Study Group, was successfully organized. The session convened a diverse group of global experts in quantitative MRI for RT, who presented the latest research on the technical development and clinical translation of various quantitative MRI techniques for biology-guided RT planning and delivery. The session highlighted clinical needs and a variety of MRI techniques, including MR spectroscopic imaging, oxygen-enhanced MRI, four-dimensional MR fingerprinting, dynamic contrast-enhanced MRI, and Xe MRI. Additionally, technical aspects and challenges for clinical translation of quantitative MRI into biology-guided RT were presented, both in the context of RT planning and adaptation. This article summarizes the progress made in this emerging field, identifies key challenges that need to be addressed, and outlines areas for future research. These insights are crucial for the integration of quantitative MRI techniques into RT clinical practice, ultimately aiming to improve patient outcomes through more personalized RT approaches.