Recently, the application of MRI in Radiotherapy is growing. This started with the application of MRI in the radiotherapy treatment planning process. The superb soft tissue contrast of MRI is being used for better tumour delineation while functional MRI like DWI and DCE, is used for better tumour characterization and treatment response assessment (Lagendijk et al. 2014). In 2000 Lagendijk and Bakker proposed the use of an MRI system for on-line and real time guidance of the actual treatment process itself. Such an application requires full integration of the MRI with the treatment machine. Several integrated systems are now under development (ViewRay®, Fallone et al. 2009, Constantin et al. 2011).At our department, in close collaboration with Elekta and Philips, an 1.5T MRI in combination with a 7 MV IMRT accelerator is being developed (Lagendijk et al. 2008). To prevent the magnetic coupling between the accelerator and the MRI system, this system uses a modified active shielding magnet system to create a zero magnetic field in a toroid closely around the magnet (Overweg et al. 2009). In this midplane zone the accelerator is placed preventing coupling between the two systems (figure 1). It was shown with an experimental prototype (Raaymakers et al. 2008) that both systems fully function and work completely independent. As such diagnostic quality MRI is becoming available at the actual moment of treatment, allowing direct soft tissue visualisation for targeting and tracking.Greatest clinical expectations are for those tumour locations where present conebeam CT position verifications fails. This are especially the soft tissue locations in pelvic, abdomen and mediastinum, with tumours of the oesophagus, rectum, kidney, liver, etc. The clinical introduction of the MRI linac is being shaped using an international consortium of major radiotherapy centres, among which the UMC Utrecht, MD Anderson, NKI-AvL, MCW Milwaukee, Sunnybrook, Royal Marsden and Christie.Having the anatomy available at the actual treatment, it is clear that the optimal dose distribution can only be obtained if this anatomy is the basis of a new treatment plan. This implies on-line and real time treatment planning on the 3D MRI taken at the moment of treatment (Kontaxis et al. 2015, Lagendijk et al. 2014). Essential is that the body target locations deform and move with breathing, bladder filling, stomach filling, peristaltics, etc. This requires MRI techniques which can follow movements but also which provide optimal image quality in slowly moving structures. As example, multi-dimensional imaging, combining 1D, 2D and 3D imaging with 4D tissue models can provide the information required (Stemkens et al. 2015), but further progress is needed in this field.The use of MRI for therapy guidance is not just limited to the MRI linac design. Modern brachytherapy, the insertion into the tumour of radioactive sources, is becoming fully MRI guided. The high dose rate (HDR) treatment of cervix tumours is already the international standard (Potter et al. 2011, Nomden et al. 2013), while the MRI guided HDR treatment of recurrent prostate tumours is being introduced (Peters et al. 2014).At the UMC Utrecht this work on MRI guided radiotherapy is being focused at our Centre for Image Sciences. This unique Centre, with its focus on MRI, employs over 150 PhD students and covers the full range from fundamental physics research till the actual clinical studies. MRI Therapy guidance is being exploited for the MRI linac, MRI brachytherapy, MRI HIFU and MRI Holmium radioembolization (Merckel et al. 2016, Smits et al. 2012).