The use of magnetic resonance (MR) imaging in radiation oncology is improving dramatically. This review article discusses the necessity of image guidance and how MR finds a significant place in radiotherapy planning and delivery. The challenges to and current solutions for an in-house MR simulation, dedicated MR simulator, estimation of electron density using MR image sets and development of MR-compatible treatment planning systems are presented. This article also reviews the feasibility, advantages and limitations of MR image-guided radiation therapy (MR-IGRT) and its drive toward the integration of radiation beams with MR technology. Specifications of Co-60 MR technology and three other MR-linac projects worldwide are presented. Online and real-time MR guidance is also discussed. © 2017, Japan Radiological Society.