Proton therapy and MRI-Linacs are two of the most exciting and fast growing technologies in radiation oncology. With over 100 MRI-Linacs and 100 proton therapy centres either in operation or under construction, an integrated approach that brings together the excellent soft tissue imaging of MRI with the superior dose conformity of proton therapy is compelling. The promise of MRI-guided proton therapy has prompted multiple research studies and the building of two pre-clinical experimental systems, taking us closer to realisation of this technology. Patients who would benefit most are those whose cancers have substantial tumour motion or anatomical variation, and those who are currently unable to receive safe dose-escalation due to nearby critical structures. MRI-guided proton therapy could allow more patients with pancreatic cancer, central lung cancer and oligo-metastatic cancers in the upper abdomen (e.g. liver and adrenal) to safely receive escalated curative doses. Head and neck, lung, brain and cervix cancers, where treatment accuracy is affected by inter-fraction tumour changes such as tumour regression or changing oedema, or normal anatomy variations, would also benefit from MRI-guidance. There will be new options to improve cure by functional MRI-guided biologically adapted proton therapy. This review focuses on the clinical aspects of MRI-guided proton therapy. We describe the clinical challenges in proton therapy and the clinical benefits from the addition of MRI-guidance. We provide updates on the design and beam modelling of in-line and perpendicular MRI-guided proton therapy systems, and a roadmap to clinical implementation.